sprayberry academy of radio - nd-3 - lines of force

7/28/2019 Sprayberry Academy of Radio - ND-3 - Lines of Force

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LINES OF FORCE

MR. R. G. fARRAH91 3 E>ther StreetVancouver. Washington


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NEW CONCEPTS AND TERMSThe. . . ._ whlc l l - prn ted In thb lena--' wlwell. . . . You will Hlld IWIW.... herein INIWY of the 4uestloM OW IIIII. . . . . . . . . . . . ,.. ....... . ed ................... child, , uw,... tint . . . . . . or eomr=M ay ,..... ef .W. Ienon - cctuUy t.w . . . . . . , , dH .....' - lwctha wllldo .. , peon Hilde Ia ... . . . , . , .etlan ef 1 .;I Pie f IAI 1 part ef your redlo-teiRwislen .....,lea cad lcter, Ia yol l t ' a,_,. , . . wiU Had thct I epKicl RI W lcagucge mtQt be lumed. n11league.. Ia bc11d on IIIW coiiCIJIII. There ere mcny thl . . . Iarcdlo end tell\'llloa which - pacullcr to th- eubjecll. n.. ,c IIIW lent.... II dl\'eloplag clmoct dcUy, new words ere. . . . . . . - .... -. . . . . . . _,.. witll ,.. clld ... . . . . . . l r . . . . . . . . . . . telk - - .............. ......l n tWe l . , . . w ~ a ....... ...., .... _ .............not c upar . . . . . . . . . . . . ta Preach or C:W- b .t I t -. . . . . . . _,. rnlciiAII _,. .......... __ , . . . . , . . . _ , _ . . ef - udn., . , . , . . ._ worda will c o - . . yow-at . . . . wltll I . . . . Qlllled .. thcH IIIW I I UJII - IIIey.... I I l l ly . . . lime JN ..,. II phted ... 1181... JNd .... a .. . I l l I M I ef . . . . - h IHIIJI ...U ef ....... JIF' I II lui ef ...... Iaili I ha npck , . ._'&

.. . . . . . . ............... 115 . ,........... ....... ,.. ..... ,.. .... .,-.. ............ -.., ....................."' .. ~ , . . .......................


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The- ohO'P photo i l l u f i l r a h ~ ! ~ t tom' of th co laboratory IUL t"QUtPmtnf utd to tHI the part s and wlrlnof mod .rn rtrcoivt'l" ( ' l r ~ u t t

HowAppliedLines ofin Radio

force Areand Television

LESSON ND-3You are now entering a pha:-;e of."ltudy which is most interesting andfaHcinating-in fact, when youth01oughly comprehend the fullsignificance of the actions set forth

in this Jesson, you will have a goodu n d e r > ~ l a n d i n g of more of the basicprinciples which make radio andt


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''OBLONG LENGTHOF MAGNETICIRON I I iG.1.1

which prove that magnetismpart of the universe, just as aira part of it. With the properground obtained by the studynatural magnet, you may then Pl'6oceed with the study of Dl&Petil18(produced by electrical means aa ft)is utilized in radio and t e l ~circuits.NATURAL MAGNETS

Long ago in Asia Minor (in theprovince of Magnesia) a certabairon ore having very peculiarproperties was found. I t was die-covered that this particular orewould act on certain other m e t a l ~at a 4is.. It- is not known who1lr8t 1 ~ itbJa pariicular action,but i t - . bean ol>served sinee theearliest tim& From the name ofthe provblce of t ~ the word-magnet is obtained. Thus, whencerte,in. ~ ubibtt the charao-teristic 6f aifectfng other objects

II ISOUTH POLE

NORTH POl-E

IFIG.2.lMAGNETIC FIELD ABOUTTHE EARTH


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at a distance, they are called mar-nets, and, of course, from magnetthe words magnetism and magneticare derived.

I t has been found that if an oblong length of magnetic iron is sus-pended in the center by a threadso that the iron is free to rotate itwill alway;:; exhibit a very defin'itecharacteristic. One end of the ironwill (when it comes to rest) pointto the approx-imate geographicalNorth Pole of the earth. The otherend will point towards the approximate South Pole of the earth. (SeeFig. 1.) This proves that the polesof the earth have a very definitepower over the magnetic iron.Scienti:;ts think of this power aslines of [o1ce. That is, it seems thatthese lines of force exist betweenthe North and South Poles of theearth spreading out so as to coverthe whole surface of the earth. Aconcept of these lines is shown inFig. 2. The area over which thelines of force ctct is called mag-netic field, or more commonly, thisis shortened to fielcl.

I t has been proved that thesemagnetic lines of force of the earthacting on (or going through) theMagnesian iron ore deposit magnetize the iron ore, and by this action cause the mineral to also exhibit magnetic lines of force. Thatis the explanation of how this par-ticular mineral or any other, obtains its appnrent natural magneticeffects. I t is important to note atthis point that the natural magneticlines of force from the earth, inpassing through great masses ofiron, imparts to the iron a permanent magnetic effect. All thishas been accomplished by invisiblemagnetic lines of force passing 3

~ h r o u ~ h the iron. As a result the~ r o n Itself becomes a magnet1!1 turn radiates its own m a g n : ~ ~

h n ~ s of o ~ c e . There is another wayof Impartmg magnetism from onemagnetic substance to another. I fa small metal tool such as a screwdriver is stroked briskly severaltimes across a permanent bar magnet, the sc1ewdriver will take onmagnetism from the bar magnet.(See Fig. 3.) It, should be clear thatmagnetism may be imparted tosuitable metals through the meclium of magnetic lines of force orby mechanical action is illustratedin Fig. 3, where the screwd1iver isrubbed across the length of the barmagnet. Further on it will beshown how metal may be magnetized electrically.

The question naturally comes upas to why the Magnesian iron oreshows magnetic effects (due to theearth's lines of force) while otheriron ore deposits may not show thesame effect. The answer to this is,that the magnetic field of the earthis relatively weak and will not showmeasurable effect on s m ~ l l l , impureiron ore deposits. The effect is bestnoticed where there is a large con-

SCREWDRIVERRUBBED BACKAND FORTHOVER THELENGTH OF ABAR MAGNET

N


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'These expressions are oftenshortened to simply nor thlN) pole and south (S)vole. Thus, every magnethas a north and south pole.

' '. ............ __---\ ' ..... .. .,.,. .,- /' , , ., I' ..... ....- _, .;1' , ;.. .,.....___

The first compass wasjust such a device as shownin Fig. 1. The basic form ofthe compass has not changedappreciably to this day. Ancient sailors used the compas" for navigation for

MAGNETIC FIELD ABOUT A BAR MAGNET

many years. During all this timenothing was known about the magnetic sliver of iron ore except thatit would always indicate North andSouth.

Since a magnet exhibits this property of ath-action by the poles ofthe earth, a very important law ofmagnetism was logically deduced.Further confirmation of this lawconsisted in placing two magnets11ea r each other. The result wasthat when the north poles of themagnets were near each other, themagnets pushed each other away.When the south poles were broughttogether a similar result obtained.However, upon bringing the southpole of one magnet near the northpole of the other. it was discoveredthat the magnets came together.The important magnetic law whichsums up this demonstration, andapplies to all magnets and magnetic poles is :1. Like magnetic poles repel oneanother.2. Unlike magnelic po les attractone a nother.

little practical value. However,from the study of the simple natural magRet, man bas learned to create large, strong, useful magnets.This is usually accomplished electrically, as you will learn as youprogress with your studies. Butbefore going into this, it will benecessary for you to study moreabout the laws which govern magnetic lines of force.

By cateful research and study ithas been found that the lines offorce about a magnet have a verydefinite path and extend in certaindirections. Inside a straight barmagnet, the lines of force extendfrom the south to the north pole.This means that they leave themagnet at the north pole and reenter at the south pole. Thus, thereis a continuous mass of magneticlines in and about a permanentmagnet all the time. By permanent magnet is meant one like theMagnesia iron ore - one thatpermanently shows magnetic effects. There are temporary magnets which ~ r e called electrom.ag-1/.e.ts-you Will study these in detail'l'hi r- iR the basic law of magnetic 1 tet.repulsion and attraction, and When magnetic lines of forceshould be kept in mind at all times about a magnet are referred to

when studying magnetism. reference is made to just a f e ~Magnesian iron is called a na- lines. (See the lines of force in Fig.tural magnet. Such a magnet has 4.) Likewise, when these are repre-5


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FIG. 5sented on paper, only a few lines onstrations have been made overare shown. But this does not 1epre- and over again, so it is not neeessent actual conditions. Even if a sary for you to make the set-ups tolarge number of lines could be prove these things unless you wishshown on paper, this would serve to do so for your own personal sat-only to indicate direction and in- isfaction.tensity. The important point you Natural magnets are often refer-should understand is the fact that red to as lodestones, meaning leadmagnetism is intangible. The near- ing stones (used by the ancients tDest approach to making it tangible lead them in the right direetion}.is to show lines on paper which A natural lodestone or an ordinaryrepresent this force. Magnetism is bar magnet and a few iron filingsnot a thing that you can divide in- may be ued to demonstrate the at-to unit lines. The expression lines tracting qualities of magnetiJJm.of force is usP.d simply for conven- Iron filings are tiny bits of iron orlenee aU other electrical litera- steel obtainable from any me}lhrAtare ia written from the same view- shop where drill pres'es, . . .poillt--tJms, the expression is not etc., are used. Half a pound ofnew. Jlaanetiam is a continuous filings is entirely sufficient torunblterrupted force in its normal experiment. If you place a&tate, extending out in all diree- stone or small bar magnettioDe, butt of C01.U'8e, limited in its the filings, you will And:btteuity at a dWance. filings are immediatelyDBKON8'.l'RATING THE LINES to the magnet.. FurtherOF FOR find the areatest *U I 'CB two ends of the'.l'ber& are several ellllical ex. there will be a & ' J . " f i ~which m&J be performed atlckiq to

~ e z n o u t r a t e the proof ot you ean maOfb're at torce. Such . . . map.q tz.o:mt


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tiling5. Figure 5 shows this effectin detail.There is n good reason why thetiling .: cling to the ends of the magnet and not so well to its center.The metal oj which the. bar magnetis made is a good conductor of -magnetism, wlzerea.s air is not. Thuswithin the length of the bar magnet the l i n e ~ of force have a goodpath over which they can travelwith little opposition. At the endsof the magnet the lines of forcespread out over a wide area because there is no good conductingmedium to confine them to a givenpath. I t should be clear, then, thatat the ends of the bar magnet thelines of force converge as they entel' or leave the magnet, and atthese ends there is the greatestconcentration of lines of force anywhere outside of the magnet itself.Thus, attraction at these points isgreatest and that is the reasonthere is the greatest mass of filingsat the magnet ends. Inside the barmagnet the lines of force are concentrated along the iron path andthere is little or no external magnetic effect along the length of themagnet.

In Fig. 4 you saw au imaginarygrouping of magnetic lines of forcein and about a bar magnet. An im-portant point to remembe1 aboutfhiB is thaf the rnag11etic lines of

f o ~ c e external to the magnet- areexactly equal to those within themagilef. Those outside of the magnet spread out in ever-wideningcircles growing weaker and lessdense as they proceed away fromthe source.

ELECTROMAGNETISMIn order to get a fundamental understanding of magnetism, it is

n e c t s ~ a r y to study the magnetismassociated with electrons anda t ~ m s . From this study you canbmld up a concrete conception ofmagnetism which will go a longway in helping you to understandother radio principles. You will nodoubt 1ea1ize the significance ofthis as progress is made from onelesson to another.

7

An ~ > l e c t r o n in motion has a magnetic field about it . There is nogetting away from this fact--whenan electron moves it sets up a magnetic field. The field is strongest ator near the electron and decreasesat greater distances from the electron. This magnetic field about amoving electron is shown in Fig. 6.Note that as the distance from theelectron increases, the lines representing magnetic force become further apart- this is a practicalmethod of illustrating the decreasing intensity at further distancesf1om the electron. In Fig. 6 the electron is represented as coming out ofthe paper. Note that the electriclines of force are polarized in aclockwise direction. If the speed ofthe electron is increased, the in en------ ....../ '

/ ---........ '/ '-.. ~I / _.-........_ ""' \I 1;/ .--... ' \ \ \I ( //--.. \ \ \ \I I ( t{ ) \ \ I I\ \ \ \ '- - - ~ ~ ) I I\ \ " ...__ / /\ """ ....__/ / I' \ '- / /

"' --- /........... /----FIG.6


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THUMBINDICATESDIRECTION OFCURRENT INCONDUCTOR

LEFTHAND

FINGERSINDICATEDIRECTIONOF fiELD

(FIG. 7.1s1ty of the magnetic field increases- o f course, the field will then bemeasurable at a greater distancefrom the electron, also. The magnet"c field which accompanies a tra-veling electron has a definite direction with respect to the electron. Byexperiment it has been proven thatthe magnetic field of an electron inmotion is at right angles to the direction in which the electron ismoving. Thus if an electron is moving along a horizon tal straight line,the magnetic field will lie in a vertical plane. In Fig. 6 this is clearlyshown by the arrow heads indicatIng the direction of the lines offorce representing the magneticfield.

You may further illustrate thisrelationship by pushing a pencilthrough a piece of paper. The pen-eil represents the Hne of electrontravel; the magnetic linea would allBe on the paper. A well-known rulewiD always estabJ"sh the directionof the magnetic lines of force abouta slnalewire conductor-if you willremember to apply it. I t is calledthe left Jumd rule and applies in

8

every ease, i f you comider electnmcurrent flow to be from negati1Je topositive. To apply this rule, useyour left hand, as shown in Fig. 7.Imagine your thumb to point in thedirection of electron current flowand grasp an imaginary wire asshown. Your fingers then point inthe direction which the magneticlines of force tal'e in circling aboutthe wire.Every electron in motion then,carries its magnetic field alongwith it. You know that in an atomthere are several electrons spinning

about the nucleus. Each electronhas its own magnetic field. Referto Fig. 8. Here you see several electrons in their orbits about the nucleus of an atom, each with its field .Now, since they are all travelingin the same direction you see thatat the center of the orbits, the magnetic fields add. Thus a singleatom may be magnetic.Now consider two electrons moving together in a conductor. Bothwill have magnetic fields u:hich willmerge or combine somewhat as pic-tured in Fig. 9A. Three electrons

will have an even greate1 combining effect, the total field wiU ap-

ICUMULATIVE EFFECT OFELECTRONS TO BUILD AMAGNETIC FIELD THROUGHTHE NUCLEUSIF'IG.BJ


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small that it cannot be measured.To have a basis from which to determine the relation between thecurrent intensity and the magneticfield produced by it, a point maybe chosen at a definite distancefrom the wire, say one inch. Byusing this system it has been foundthat the magnetic field is exactlyproporti'otl.al to the current flow inthe w r , providing no magneticmaterials are present. If the current is doubled, the force of thefield is also doubled, or if the current is reduced ten times, the fieldIS in like manner reduced ten timestn intensity. Thus there is a trueproportion between current flowand magnetism.

I t is impossible for an electron tobe in motion without its magneticfield accompanying it. I f movingelectrons (comprising current flow)are stopped, the magnetic field atthe source must also stop. I f a magnetic field is created by electronsin motion, it will tend to make allelectrons in the area of influencemove. Just as surely as the elec-tron tn mot on wz1l produce a. ma.g-Mtic field, a. magnetic field in mo-Ciot& will tend to produce movinguag electrons, or in other wordseause an electric current to flow.

CREATING ELECTRO-MAGNETIC FIELDS BY MEANSOF COILSHere i an tmportant fact-besure to remember tt. In a magneticfteld of fiud tttteftsitv, there is no

'IIUJI11U1tic motion. Thus a magneticfteld produced by the flow of di-rect cnwrent is fixed and steady -its presaure or area of influence ismerely being exerted in a definiteclirection. With AC, on the other

hand, the magnetic lines of forcechange their direction and of coursein this case there is motion or achanging magnetic field-but moreabout this later. Do not confuse thearrows used in Fig. 6 through 9(which indicate the direction of themagnetic force), with the idea ofcontinuous motion. This idea of afixed magnetic field is analogous toa compressed coil spring. A coilspring may be compressed and mayremain quite stationary, and yetthe direction in which its force isexerted may be represented with anarrow. In many ways a magneticfie ld in and around a straightlength of wire, such as described inthe foregoing, will be useful, butfor other uses there must be magnetic fields of different kinds and ofdifferent i n t e n s i t i ~ .

In further consideration of this,suppose a length of wire is loopedas shown in Fig. 10. I f this is donethere will be two lengths of wirepassing close to one another, bothhaving magnetic fields, and it willbe noticed that for a short distancethe two loops of wire are parallel.Current is flowing in the two ad-

SHOWING HOW MAGNETISMMULTIPLIES WHEN A LOOP

OF WIRE IS USED11G.1o.1

10


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nt lengths of the \\ 1re in theJ&C .same d rect10n.' ! l : o ' ~ in exactly the ame mannerthat the two individual fields of apair of electrons combine to form aingle field, as shown in Fig. 9A,the t \ \O fields about the two lengthsof "ire will also combme as shownin Fig. 10.If you should wind more turns1 re alongside one another, all

oJ thr separate fields of the indi-aual wires tcill combine or mergeo tiler forming one big magneticfi d Several turns of wire aresh1m n in cross section form in Fig.

11. ns though all of them were cutin the center. By forming a coil(v;rapping several turns together),various lengths of wire will all contribute to the forming of a singlefield (which is very strong in thecenter of the coil) and all of thelines of force will act in the samedirection, just as explained for theelectrons in Fig. 9.While magnetism also exists outside of the coil and away from it ,1t spreads over so much space thatit is not very strong or intense at

II

I , ELECTRONI F'LOWII I,

IIII ,' II I I

/ I, '- ", / I I I _,,,,,, .. I !G 111J'"' ,, ' I \ .. - I I I 'SHOWING THE TOTAL MAGNETICFIELD FOR A NUMBER OFLOOPS FORMING A COIL

. - ' J - - - - ~I I..' ''. 'o\ :.-

SHOWING HOWMAGNETISM WILL FORMALL AROUND A COILCARRYING CURRENTIF'JG .12l

any one place. The total outsidefield extending indefinitely fromthe coil, however, is equal to thetotal inside field in intensity. Eachtum of wire that is added to thiacoil will add to the total magne.tism.

Consider another type of coilwith a great many turns of wirewound layer after layer as shownin Fig. 12. I f it has 50 times thenumber of turns of wire that theone in Fig. 11 has, it will have amagnetic field 50 times the strengthof the coil in Fig. 11, provided, ofcourse, that the same value of current is fiowing in both coils.

You are now prepared to con-sider the effect that a magneticfield will have on various other BUbstances. Although i t is necessary toget down to considerable detail inthe following study, it will allnaturally follow the principles already covered in this Ieason. I f youu


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understand the foregoing, the folIo\\ ing will not be at all difficult.

THE MAGNETIC NATUREOF METALS

material in n nomtal state is suchthat no lotnl magnetism will be developed. This is because the position of lhe atoms (with their electron orbit:;) will not be disturbed.In thtse ?WII-magllciic substances1he magnetism is contpletely bal-ancC'Il out, dllC to the fact that the'IICI'IIY of tht rlecfl'fllls i1l one di-rPctlou i:{ equal to that in all otherdirccfitMs so that no external mag-t iel ism exist:;. Therefore, whenmaguetic lines of force are impressed through them no change ismade in the material. The magneticforce will pass right through theempty space (ether) between theatoms of the non-magnetic subst:mce "ithou\ changing the atomsf the substance in anyway. If thesame magnetic field is impressedupon iron or some other magnetic.mbstnnce, the behavior of themolecules and atoms will be quitetlifl't>tent. Remember this point, asi t will be useful to you in the future.MAGNET IC COILS AND CORES

Ev>ry known substance is mnrleup of electrons which are in \'ariousstat


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/ . " , \ ~ l?; .. ~- ~ J . ' \ -'" ;... --.!. :\ ~ ;.!5 - ~ ~ ir 64 , ~ %7 -t ;:; z;< -t .:t- * .:\. ~ % ~ ~

ORDINARY SUBSTANCE OR MAGNETICSUBSTANCE BEFORE MAGNETIZING.

MAGNETIC SUBSTANCE AFTER MAGNETIZING @SHOWING HOW THE ELECTRON ORBITS ARELINED UP- NOTE THAT THE ELECTRONS AREALL REVOLVING IN THE SAME DIRECTION.

SUBDIVISION OF A MAGNET SHOWINGHOW EACH SEGMENT BECOMESA SEPARATE MAGNET

AND THAT THEMAGNETIZATION IS CONTINUOUSIFIG.13.I

s u . b s t a r ~ c e before magnetizing isshown in Fig. 13A. This representstre condition of its atoms beforeany extemal magnetism acts on it.The short arrows represent themagnetic field of each atom. Whilei t is true that every atom directed\'ertically or evc11 approximatelyso, adds (its i11tenc;ity) to the totalexternal magnetism, yet every onepointing down or approximately so,subtracts (its own intensity) fromthe total field. Thus, before any external magnetism 1-s applied thesubstance has no e;dernal ma[lneticeffects. Non-magnetic substancesare not like thi!=i, because the atoms

13

of which they are composed arenon-magnetic to begin with, awooden coil form, for instance.Materials in this condition neitherhinder nor help the magnetic fieldapplied through them.

If you should place a piece of:;oft iron, steel, cobalt, or nickelthrough the coil of Fig. 12 while itis carrying current, somethingquite different will happen. Themagnetism produced by the coilcarrying a current will rotate theorbits of many of the electrons ofthe magnetic metal substances sothat their effects will be impressedin one direction-namely, the di-


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1 n of th e rnal f1eld actmgon them. See F1g. 13B. For balancedatom in a magnetic substance,ome of the electrons may be re-versed in their orbits so that theatom will be sensitive to magneticinfluence. The external or appliedmagnetism will then tend to turnth orbits so that the separatedirection of the applied lines ofmagnetic influence of each atomwill be aligned or impressed in thedirection of the applied lines offorce. The force which builds magn tism by aligning the influencesof moving electrons is called ma.g-tomotive force. Using a piece ofoft iron, the field originally causedby the coil alone will be intensified many thousands of times. Thisis because the soft iron is a betterconductor of magnetic lines of forcethan air. The iron core concentratesthe lines of force in a minimumarea. All of this is due to the cho:ngen poBition of many of the orbits ofthe electrons in the iron. Moreover,if the iron is removed from the coilsome of the atoms of the iron willtemporarily remain in their newpomtions and the iron will displayeamemagnetic properties as the

hard steel is used for this experiment JDOBt of the atoms will remain their new positions and thelteel will be permanently magna-sed. 'l'bua JOU eee how electricitymay be Ull8ll to mapet lze a barmapet. Tbia tne also for somefew other aabilf!nc, neb as eo-bait, Jdekel UM1 1lfauW oxygen, butto a lelller de8ree. IPla l iB showsa paphic ~ t l o n of thepaBtioDB of the atom& flf the iron. . . after tUJ: haw been IOb-jlztecJ to az ?! 1.

u

You can further demonstratehat this condition exists through

out the entire length of the iron orsteel by cutting a bar magnet intoseveral pieces. Each piece will be-come a separate magnet as shownin Fig. 13C. This is further proof ofthe inherent magnetic nature of theatom and molecule, because nomatter how many times you dividethe bar magnet in Fig. 13C and nomatter how small the individualpieces become, each piece will stillshow the effects of a north andsouth pole.

This individual nature of a limited number of substances seems tohave a definite relation to mechanical motion. If you hold a magneticsubstance in a magnetic field,either that of the earth or one produced by a coil of wire, and tap itsharply with a hammer (create mo-tion) the tapping seems to aid themolecules in assuming their correctpositions. On the other hand, i f youmagnetize a piece of steel, take itaway from the field used for theprocess and then tap it, it will losesome of its magnetism.I f you bring a bar magnet near apiece of iron such as a nail, itsfield will immediately extend intothe nail, and will align many of the

molecules in the nail. The northpole of one magnet will attract thesouth pole of another-similar tothe law of electrons and protonslike charges repel, unlike chargeSattract. Thus like magnetic polesrepel tl1ld unlike magnetic poluGttrGCt. The nail or a piece of ironor steel will, therefore, be drawnto the magnet pole, as pictured inFig. 14. The other end of the maa-net will attract iron for the D IPreason, but in this ease, id1 of the


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MAGNET

SHOWING WHATHAPPENS WHEN AMAGNET HOLDS ANAILIFIG.\4.1

s poles of the atoms of the magnet\Vill be attracting the N poles ofthe smaller piece of metal. Thepiece of iron or steel will b e c o m ~ amagnet itself, at first strong wh1leunder the direct influence of themagnet and then weaker as isremoved-but as was explamedmany of the atoms will now ?;ain-tain theit newly acqui'red posttwns.

An ordinary piece of steel willbecome magnetic if placed in linewith the N-S poles of the earth. I fa steel needle is magnetized andftcaterl on a cork or piece of woodin water it will turn its poles toward the poles of the earth. Inother words it will act as a compass.

Both permanent and electromagnets are used widely in radio; theseuse.:; are b a : - ~ e d on the foregoing explanation of how iro11 or steel isaffected by magnetism.

ELECTHON MOTION IN AMAGNETIC FIELDConsider now some of the morecommon applications of a g 1 ~ e t i s memployed in radio. Y o ~ w1ll seehow the force of magnetism, called

magnetomotive force, can be eonverted to electrical force calledelectromotive force, or voltage. Also vou will see that electric andmagnetic energies can be o n v e r ~ e dinto mechanical energy and viCeversa. 'I'his study is most important

l)Pcause the principles are em.plo.rcd ove1 and over again in radioand televiswn circuits. So makesure you unilerstnnd these pl'in-ciples as you advance with yourstudies.

Wherl) a conductive path, suchas a copper wh-e circuit, is providedfor a voltage, electrons will movein the whc, forming a current ofelectrons, or simply a ftow of current. I t has been explained that,clue to the nature of an electron,when it. moves it creates a magneticfield. Remember also that an electron is always in motion. \Vhen itis not traveling along a conductorby jumping from one atom toanother, it is revolving around thenucleus of an atom at an extremelyhigh speed estimated at severalmillion tcvolutions per second.Thus, in the normal state the electton always has its magnetic field.Howeve1, \Vhen there is no voltageto shift electrons away from theiratoms, all of these fields are invarious directions (as explainedformerly) giving no total externalmagnetic field. When a voltage isapvlied to a conductor, electronsare caused ttJ flow through the conductor. I t is these electrons whicher.:!ate the useful magnetic fields intadio. The magnetism due to therandom motion of the electron intheir individual orbits is not included in practical considerations.So, for the remainder of this leRson,only electron motion throug-h aconductor will be considered.

~ o w assume an electron lo bemoving through a magnetic field ~ nthe direction indicated by the soh_darrow (A) in Fig. 15. The magneticlines of force will p a s : : ~ from thenorth pole to the south pole shown

15


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cDIRECTION 1ELECTRON IN WHICHIS URGED

thP d ~ e c t i ~ m indicateu by the arrowB, wh1ch IS the resulting direetioof. travel. You are familiar w i t ~this p ~ e n o m e n o n . I f you throwMAGNET stone, has two velocities, one du:

to gravity and the other due to thforce you exert on it. The stone willp r o c ~ e d . outward and downward,

DIRECTION INWHICH ELECTRON ISORIGINALLY F'LOWINGFIG. IS

N

n the figure. Now consider themagnetic lines due to the electronmotion. They are in a circular pat-tern, vertical to the direction offlow (A) of the electron. Above theelectron, the lines of force due toelectron motion are in an oppositedirection to the lines produced bythe magnetic poles. The net resultis that the forces tend to cancel,leaving a weaker field above theelectron. Below the electron, thel'nea of force due to electron flowthose of themagnetic poles arethe same direction. This resultsa additive effect, which pro-a stronger force below theThere is now an unbal-fllure, with a greaterl f l . _,.etr.i)n than there

bat i f twoobJM; the object~ o f the.. J D J ' j g . l 6weed upWard~ b y the

contmumg to travel away from youuntil it strikes the ground. Theelectron acts in a similar manner!n this case bt!ing forced upwardm addition to its original direction.You have here mechanical force, ormotion, being produced by an elec-tron passing through a magneticfield . This is very important-intelevision you will see a very prac-t ~ c a l application of this basic prin-Ciple. In fact, without this principlethe completely electronic televisionsystem, so common today, would beabsolutely impossible.

The direction in which a movingelectron is forced by a magneticfield is perpendicular to the origin-al direction in which the electronwas traveling, and perpendicular tothe direction of the lines of force ofthe magnetic field. This is a veryimportant statement as it coversthe deflection of an electron mov-ing in a magnetic field, regardlessof the circumstances.THE MOTOR PRINCIPLENow take the free electron dis-

cussed in the foregoing paragraphs


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Ref rnng back to Fig 17, .) oucan reahze that all of the free elec-tron m t n luct r ' ' 11 be af-fec:ted n t the s m way nsth 1 gl ron mentioned md. u n he g 1erator principle.T c mb1 ed mo\ ement of all oft ron \\ 111 form an electron c rret flo\\ m the conductor.\\ n an electron or group of electr 1 di placed-that i!(, movedrom one place to another, theyke the1r charges with them andadd a negathe charge to the m ~ -

r"al in \\ h1ch they are found. Th1si one \\ay in which electricity canbe cau ed to exist in a conductor bymeans of magnetic lines of force.Th"s is an elementary principle,nd later on it will be shown howrae 1cal u e is made of this prinip e.

When electrons are removedrom one place, that same place b ~ -comes pos1tivels c h a r g ~ , and 1slaced in such a conditiOn as tocause 1t to attract electrons. Forexample, if you utilize the principleFig. 17 and cause a number oflectron to move toward point B

-r--r .. ... ~ ..... . - A. . . . . . . . . . . ... . .. . . . . . .. .

E 0FIG. 18Referring again to Fig. 18, if

the electrons are prevented fromdrifting back to the left by the voltage created in the conductor, andan e:\:ternal circuit iR connectedsuch as the wire C, D, E, F and G,the electrons will press themselvesout into the external circuit andforce their way around the circuitand back to the left end, as thereare fewer of them in the wire atthe left end. It is assumed herethat, before connecting the externalcircuit of B-G of Fig. 18, theelectrons in the connecting circuitwere originally distributed as forFig. 18A. As soon as some of theelectrons move from B to C, in Fig.18 those at C will become crowdedand since they cannot move againstthe pressure of electrons at B, theywill move to a less crowded sectionof the wire at D. Those at D wi!Jin turn move along to E, etc. untilthey are all equally distributed inthe external circuit and the con-ductor G-B. No more voltage willthen exist anywhere along the circuit. Thia is the action which takesplace in a conductor when magne-tic lines of force are caused to induce a voltage in the conductor bythe motion of the conductorthrough the lines of force.

the conductor (in which theywere originally equally distributed,aa Fig. 18A), a voltage will bee.tabliahed between the ends of theconductor. Since more of the electrons aecumulate at end B of theconductor i t ia negative, as eachelectron hall a negative charge. (SeeG-B of Fig. 18). The other end ispositive because IDilDJr of the atomshave lost their electrons and hencebecome positive. This ia the condition in a conductor when magneticlines of force an ltlled t1oeauae currem flow in a eireaft in which noCUI IeDt oriP1aJly existed.

There is a rule to be used inshowing the direction of currentflow in a conductor which is mov-18


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mg through a magnetic field. Thisrule is called the left-hand gener-ator rule and is similar to the right-hand motor rule. Using the lefthand, the thumb represents the direction of motion of the conductor,the middle finger represenl'l thedirection of the magm tic field. Theforefinger ( . l t right angles to thethumb nne! middle finger) indicates the direction of current flow.

There is another important consideration you should understandabout magnetically induced voltages; that is the importance of direction in which the conductor ismoving through the field. In Fig. 19is shown a uniform magnetic fieldrepresented by the vertical linesand spaces. The entire space is considered to be filled with magnetismand the vertical lines show that itis impressed vertically and that itis uniformly distributed. The Jinesare used to simply indicate thespace the field occupies, not as ameasure of magnetism.In this figure the end view of awire conductor is considered to beat (0 ) . Next consider what happens when it is moved in variousdirections such as (0-A), (0-B),(0-C), etc. When the conductor

moves from (0 ) to (A) it is direct-ly in line or parallel with the direction of the field. No matter howfast it moves in this direction Mooltage wiU be induced in it, be-eause it has not cut or swept acrossany lines of force.Now, I f this aame conductor ismoved from (0 ) to (F), at right

angles to the field force, it has cutthrough a section of the entire field.'-ssurne that it has moved fastcmough to have ten volts inducedinto it. Suppose now the conductor

(0 ) is moved from (0 ) to (E) , thesame dtstance as (0-F) and in theame Hme. I t has cut through onlya littl? more than 8% of the 10umts from (0 ) to (F). The value

of the induced voltage depends onthe rate at 'vhich the conductorcuts across the magnetic Held. I tshould be clear therefore, that the\"Oltage induced in this case will beonly about 8.5/10 or 857o of thevoltage induced by the motion from{0 ) to (F) . Thus only about 8.5volts would be induced.

In moving from (0 ) to (D) theconductor cuts through even fewerlines of force (about 70%), inducing only 7 volts. Similarly amovement (0-C) would induceabout 5 volts and a movement(0-B) would induce less than 2volts.To induce 10 volts by moving inthe direction (0-D), for example,the conductor would have to continue to (D') and it would have tomove the entire distance (0-D') inthe same time it formerly requiredto move from (0 ) to (F) . This isabout 30% farther, and hence theconductor would have to move

30% faster.On the other hand, to induce only

,.

, z J .,. s ., , Ifli'IG. 1819


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I volts in the conductor, moving inthe direction (0-F), it would haveto move only half way, or to (G)in the same time that it must reach(C) along the line (0-C). Althoughthe distance (0-C) is twice that of(0-G), the same voltage will beinduced in either ease.From this and the foregoing itmay be seen that the amount of induced voltage in a wire from amacnetie field depends on threethings. They are:

(1) The length of wire. A greaterlength of wire or more turns ofwire in a given coil or loop increases the induced voltage.

(2) The rate of speed at whichthe wire passes through the magnetic field, or the rate of speed atwhich the magnetic field passesthrough the wire. This may becompared roughly to the windbending a slender tree--the stronger the wind, the more the tree willbend.

(8) The direction in which thewire passes through the field. A.,.,... tJWBt pa88 through a magnetietJ1 f'igl&t angles to the lines ofto in41u:e a mazimum 'Voltage.

INDUCTION FROMA IIAGNETIC FIEI.DJiiiD be deacribed an im-'=l:fi which must be.JI 1:n order to diaplacea voltaae, the1\i.,mapel:ic Aeldrespect to

.---atatfon-!fW.ir.ful the

displace the free electrons of theconductor.Now, the magnetic field and theelectrons in the conductor have noway of knowing which is moving.

Thus, if the magnetic field is moving past the conductor, the sameresults obtain (that is, electronflow) as if the conductor were moving through the field. Refer to Fig.20. Here you see a conductor A-Bconnected to a battery through aswitch. Placed near this conductoris another conductor (C-D in Fig.20) connected to an ammeter whichindicates current flow in either direction. The instant the switch isclosed, current will flow throughthe conductor A-B in the directionA to B. A single electron is shownin Fig. 20, to illustrate the magnetic field set up about the conductoras determined by the left hand rule.Now this field cannot just begin toexist. It must start from zero andgradually build up to its full intensity. This takes only a smallfraction of a second in an actualcircuit. Now as the magnetic fielddue to electron flow builds uparound conductor A-B, it extendsoutward in al l directions from theconductor. In so extending, it cutsacros8 the conductor as indicatedby the arrow M. The effect is thesame as though CD were movedthrough a stationary fteld in thedirection indicated by the dotted

G

---DFt$20


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arroW N. Recalling thde left Ju:nd t ~ r o u g h CD in the opposite direc-ule for a generator, an assummg twn to that m which they ex[he conductor CD to be moving in- pauded. Again, it may be assumed~ t e n d of the magnetic field you that. the field is stationary and theha\c: (1) Flux (forefinger of the condudor is moving through i t - left hand) pointed directly into the the relationship of motion betweenpaper between the conductors. (2) the conductor and the field is allThrust (direction of assumed mo- that is of importance here. Thetion of the conductor CD-indi- conductor is assumed moving in thecated by the thumb) to the left. (3) direction indicated by the dottedThe elt'ctrons in the conductor CD arrow labelled 0 in Fig. 20. Applywill then move in the direction in- ing the generator (left hand) rule,dicated by the milldle finger-di- you see that the electron picturedrectly up, or from D toward C. in this figure will move do\vnward.Since lhere are millions of elec- This electron represents many electron.-; in the conductor CD, and trons. so when it moves downward:,ince many of them will be affected an electron current exists. The mein exactly the same way a::; the ter will show a deflection. Againsingle electron in the illustration the current is short lived-rememthis large number of electrons in bm i t can only be induced as longmotion will compose a measurable .as the magnetic :field and the concurrent. ductor CD are in relative motion ..As soon as the field has completelyOnce the cunent has set-up its collapsed, 110 more current will bemagnetic field about conducto1 AB, induced in CD and the meter willthe lines of force no longer expand cease to register.and cut past conductor CD. Themagnetic field has reached a stable By referring back to the discus-condition-therefore, with no rein- sion of Fig. 20 you can readily 5eetive motion behveen the conductor how voltasres are built up in CD asand the magnetic field, no electron n result of electron movement. Youmotion results in conductor CD. can see immediately that, when theThe meter then, "'i l l register a switch of Fig. 20 is closed, it willslight reading for a split second cause point D of Fig. 20 to becomeafte1 the switch is closed, then will positive with respect to point C.read zero. Now, as the switch is opened, thetielrl collapses, the electron motionNow assume the switch in Fig. is in the opposite direction (or f1om20 to be opened after having been cclosed for a short time. While the to D) thus C will become positivewilh resprct to D.switch was closed i t allowed cur-rent to flow and thjs current set up Now look hack over Fisr. 20 anda magnetic field about AB. When note the following facts. 'When thelhe switch is opened, it will prevent switch is closed, current llows fromcurrent flow. thus the magnetic A to B in the conductor connectedf i e ~ d about AB will collapse-there tn the battery. In the conductor CDb e i ~ g no electron motion, no mag- (called the secondnry) the current~ ~ t J c field c ~ n C:xist. In collapsing, tlow is f ~ o m D to C- jus t oppositee magnetic hnes of force cut to that In the primary conductor

21


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AMMETERFIG. 21

(A-B) . ): ou have read in this les-son that any electron motion is ac-companied b) a magnetic field.Thus for the :>hort time that current is induced h1 conductor CD, ittoo ,.,.ill have a magnetic field aboutit. See Fig. 21 where this is illustra-ted. The two fields, that due to thecurrent in A and that due to thecurrent in B, are in opposite di-rections, as you would naturallyexpect, !';ince the currents themselves are opposite. These pointsare very important anu will be en-larged upon later in the course, sobe sure that you understand themand remember them.

The effect which is obtained bythe use of two parallel straight con-ductors as discussed up to this pointis very, very small, and is not ofpractical value in radio circuits. Inradio work coils are used to utilizethe previously described effects insuch a way that they are caused toproduce actions which make pos-sible the fine radio transmissionand reception which is so familiartoday.

You will recall that the amountof magnetism which can be pro-duced by a coil is very much greater than that produced by a straight

wire, dm> to the accumulation ofmagneti,;m. In Fig. 22 the two coilsare hown in the same relative posi-tion so that the field of one (A)will e::-.:tend into the other (B).From your study of Fig. 20 you willrecognize how this circuit willoperate. When the switch is closed,the field about (A) will momentar-ily build up as the electrons forcetheir way through the winding, and

t h i ~ field will extend over into thecoil (B). Voltage will be inducedor created f1om magnetism in coil(B) and the meter will indicate theresulting current flow. Think thisover a bit. ~ o t e particulal'ly thatthis has caused a voltage to betransferred from one circuit to asecond circuit in which there is noba tte?y or other source of electri-cal power. This is the importantpoint of this discussion. You shouldttnderstnnd why this action is pos-Hible because this principle of voll-arle induction will be referred tooften.

In all of the preceding examples,the amount of current flow will bequite Rmall. An extremely sensitiveinstrument would be necessary togive an indication in Figs. 20, 21and 22. All of these experimentsbaNe actually been conducted in

22


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u h the arne manner a tlw." 1 h 1 E'-the,:. r p r e n Ii h \ u "I ould remember .

IRON C'ORg:::;

T R

r ~-..---..--j _ , _ : ~ - - . . - . - : .

F TG. 23IRON-- WASHER

You " i l l recall the e.XJJlmJntiOnthe mnnne1 in "hich iron orma ~ n e t i c metals acted when t iUht t a magnetic field. Tht>

tr c o magnetism on ordinary t t l at 'th d' cul'l'ent in the opposite direction.1 1 so grea 1. w1 or wary11 truments and smaU curr ent:", When electrons s1 op tlowing (duedefimte indications are ob- to opening the s\vitch) it simplym d. One "ny to place iron in means that there is no longer a flowe field of Fig. 22 is to bend se>- of electricity, and as you know,raJ large nails U shaped to fit in under such a condition there is ab th co1ls, or select a ring of iron, tendency among the a toms to reuch as a large washer or binder :-tore balance. Thus the free elecring, winding the wire on it as trons start revolving about theho\\n in Fig. The r ing set-up is atom with which t hey happen toIn far the best, as the re is iron in be associated. The fiel d t hat isthe entire magnetic path. By clos- built up in the iron of Fig. 23 vanmg the switch in Fig. 23 an enorm- ishes as qu ickly as the electronsu,o momentary field is pr oduced as :s top Jlowing in the circuit, and in

compared to the same arrangement so doi ng sweeps across all of the' ' ith air, wood or lead or any other wires, inducing large voltages innon-magnetic substance. This dem- t hem. Remember, a moving field isonstrates the princi ple on which all t he only one that wiU induce volt-electrical transformers operate. ages from one circuit to another\\ hen you come to t he practical ap- when the circuits are sta.tiona.ry.lication of t ransformers in radio, That is the reason in Figs. 22 andth s basic principle should be re- ., , that voltage in !nduced in themembered. econd circuit (B) only when curIf you were watching the meter rent is first turned on and when itneedle in an actual circuit, similar i.:; cut off. These are the only timeso Figs. 22 and 23, you would see there is a changing magnetic ftel4,that it would indicate current im- s ince the battery current is of themediately on closing the switch. DC type. While the meter indicatesThe needle would then drop to t he presence of current in circuitzero rapidly. This, of course, checks B), there will be a large apa:tkwith your understanding of mag- across the contacts of the switch innetism; you already know that the circuit (A) as it is opened. Thismagnetic field, due to the rising r esults from a large voltage produc

current, is expanding and sweeping tion at this time, due to the collape-hrough the wires of circuit (B). ing lines of force (faat rate ofAs the switch is opened, the meter change). With just the battery andneedle will kick back, indicating a switch in the circuit there wfl1 be28


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17. - of coiJa and olt . - In radio eqalpment. (A) and (8) : Tranomltlln tanlaa ooilo, B ofplq . la l) 'pe. I C) aad (D): c.a . fo r clllhrent froqaenriea with dllfennt powtr ratla Ma1 1MD.aed in eitlaer r.c:elvera or traaslttera. (): Sinl lQer choke wlnclln ..

a small spark, showing that thespark or arc is not due to the voltage of the battery. Another lessonon "nduction will treat this subjectin detail.

Radio signals are transmittedo er great distances by making themaanetic field about a wire changevery rapidly. Power is supplied toreceivers and transmitters bymeans of transformers which con-

netism which were unknowntwenty-five or fifty years ago.

ELECTROSTATIC LINESOF FORCE

I t has been mentioned beforethat the electron has its own inherent electrical charge which is present all the time, regardless ofwhether it is stationary or inmotion. This charge is of negativepolarity and is just as fundamentalas the electron itself. The electronvert power into magnetism, and can not, under any circumstances,

from magnetism back to electricity lose this charge. The electron willagain. Signals are carried from one repel another electron at a distancecircuit to another by magnetism, or, by the same token, be attractedspeakers operateby the simple laws at a distance through the influenceand principles justdescribed. Many of a positive charge-for instancenew things have been learned about the unbalanced charge of an atommagnetism in the past twenty-five which has lost free electrons due toyears. Today there are literally chemical or electrical action. Thesethousands of things done by mag- facts are definite proof of an in-

24


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1ble force at work. Too, when" ~ u consider the fact that external~ a g n e t i s m is only associated withan le tron when it is in dynamicmotion, and also that an electron\ \ 111 repel another electron no mat-r what its state of motion, youha\le further proof of another typef force ' 'hich definitely is not ofa magnetic character. Investigatorslong ago discovered this electriccl arge of the electron. They com-ared it with the magnetic effectof the electron, and were able to1rove that this electric charge wasa t{)tally different type of forcethan Magnetism. Since it wasknown that this second force isdue to the inherent, or natural electrical charge of the electron, it wasgiven the name of electrostaticcharge to distinguish it from mag-ICtic effects. The word staticmeans stationary, or standing, thusan electrostatic field is an electricfield due to stationary electrons.Actually this field exists about anelectron in motion also, but thecld is used in practice as the resultof a group of stationary electrons,and thus its name more closely fitss function. Experiment provedthat this electric charge exertedhnes of force which would act at adistance from the electrons, just asexperiment proved that there weremagnetic lines of force whichwould act similarly. Thus, lines offorce due to the electric charge ofthe electron are called electrostaticl i es of force and those due to magnetic action are, of course, calledelectromagnetic lines of force, asYou have already learned. To avoidthe use of a longer word electrostatic is usually shortened to theWord electric. When reference is

made to the electrostatic lines offorce, the term electric or staticield is usually used. This is conmon practice in the radio profes-simJ.!t has been shown previously inthis lesson that magnetic lines offorce are circular in nature, andthat they exist about an electron asin Fig. 6, or around a conductor inrings as in Fig. 7. The electric linesof force, on the other hand, exerttheir influence in eve1y direction.Figure 24 shows how the electriclines of force about an electron exert their force outward from theelectron. Note, however, that thisshows only one side view since adrawing cannot show all sides ofa sphere or ball. I f you will thinkof a perfectly round pin cushionwith pins stuck into it from everypossible angle (representing linesof electric force) you wiD get a

better mental picture of the electriclines of force about an electron.The same principles apply if youconsider a group of electrons or awire conductor either single or incoil fonn.From this explanation it should

be clear that the greatest difference between the electric and magnetic lines of force is the directionin which they make their forcesfelt; you should also remember that

FIG. 2426


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ACUT ANGLE(L!SS 1lWI ~ . ,

external mapettsm is only asaociated with the electron while i t isin m o t l ~ whereas the electriclinea of baa are ever-present.

In JOur future lessons you willhave occasion to study these twotwa. eeparately, as well as tol*"'er. For tbia reason it is mostlnpa tant that you understand thedUrwwaa between them and the-tlJnctlon in which they act.

leientlata have proved that elee-lipea of force cross magnetic

111 as of fozee at right angles whentb e two forces areasaoeiated with:!f1111J another. It is important that.,_ , have anniy fixed in your ownmind .Just what is meant by a rightaqle direction; "Jf one straight,18ta another to form equalMll18 81og!4!8 t l lQ: . , ...ua are

extending outwa rd from an e l ~tron. Around this electron (it Iaassumed to be in motion) is a lineof magnetic force-represented bythe circle. You see that the circlemay be assumed to be made up ofmany short lines. Several of theselines are illustrated in the fiaureat the points where the electriclines of force strike the circle. Thisillustrates for you the fact, statedin the preceding paragraphs, thatthe electric lines of force extendingfrom an electron are always perpendicular (at right angles) to theDl&Ciletic lines of force which existabout the electron when it is inmotion. Actually, there are thousands of magnetic and electric linesof force associated with a movingelectron, or about a wire throughwhich current is flowing. The electric lines of force move outwardaway from the electron in everydirecton, and instead of one circlerepresenting one magnetic line offorce as in Fig. 26 there are thousands of them. They are not all atthe same distance away from the


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ectron. but occupy all spaceel und the electron. These definitearo !ideation should be kept in mindqua F ' 26with reference to 1g. .y 0u will have very l ittle occasionto consider electrostatic and mag-netic lines of force together exceptwhen considering the transmissionof radio signals. However, you willhave numerous occasions to consid-er them separately. In the study ofcondensers, you will consider elec-trostatic lines of force and in thestudy of inductance, you will bemore concerned with magneticlines of force. Later lessons will bedevoted to each of the subjects.ACCUMULATED CHARGESI t is practically impossible toshow all the effects of electric linesf force around one electron. I fyou could collect a large number of

electrons you would find that theywould all repel each other as theyall have an equal and like negativecharge. Their repelling force willpress out into space in &11 direc-tions. Obviously the more electronsgathered together, the greater istheir combined pressure, which iscalled voltage. In a substance hav-ing a normal number of electronsthere is no accumulated pressurebecause with each electron there isa corresponding equal positivecharge to counteract it.For the purpose of illustrationassume that you could collect alarge number of free electrons in afiat metal plate. With a wire con-

~ t o such a metal plate ( itmarbe of as a atorag(\ tank or

as w1l1 be brought out later. To getan idea of the action of electronsunder pressure consider a flat sur-face, such as that shown in Fig. 27,which represents a segment of aplate into which many electronshave been crowded. Electrons (A )and (C) will both repel electron(B) . Under this condition, (B) can-not move because of equal pres-sures. However, if the three are re-arranged in the position shown inFig. 28 (as they would be at theend of a wire attached to a metalplate), so that (B) is crowdednear the pointed part of the sur-face, electron (A) and (C) will re-pel (B) in approximately the samedirection. The net effect is a largerepelling force on (B) in one direc-tion.

Electron (B) will tend to jumpout into the air from this sharppointed surface. I f a sufficientnumber of electrons do this, soundwill be heard and delicate violetsparks will be seen. This is called acorotta. discharge. It eo1nmGD]y oc-curs in high voltqe television cir..cuits. It will take place until theconpstion of the electrons is re-duced (such a condition ean tqt iplace in a manner to be deeiCrib!later) to a degree where thesure is not areat enough tothe corona discharge. Aamount of this action isways oecurri1Ja t ~ed 8urface where a 1arp

..... ' " ~ : 1 - u l l :


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//..-- --- ......;-" .... ,

-... ___ _., /............ --- _,.. ..,.,

VOLTAGE

/

POS. SOURCE NEG.

FIG.29electrons are collected. Many moreelectrons can be crowded togetheron a :rounded or flat surface for thisreason.

Now consider two spheres, aslihown in Fig. 29, each connected toone terminal of a large voltage.The sphere connected to the positermi.Dal of the voltage sourceup electrons to the voltage1IDtil the sphere has a deelectrons, leaving it atpotential. Theto the negative

terminal of the voltage source willhave a great number of surpluselectrons forced upon it, placiDa itat a high negative potential. B&.tween the spheres then, there exists a very strong electric field. Thiselectric field may be said to bethe result of a charge stored on thespheres. This is an important principle and you should remember it-electric charges can be storedon pieces of metal connected to avoltage source. This principle is thebasis of the condensers found in allradio and television circuits. It isnot possible to go into all the details of this subject in one lesson,for this lesson is designed merelyto acquaint you with some of thefundamental principles which youwill study further in the course.It cannot be stresed too stronglythat this Ieason is one of the moatimportant in your entire Sprayberry Course. Do not by any meanaregard it lightly, for to do so willdefeat your understanding of radioprinciples. Study this lesson again;be patient and proceed slowly.malring sure that you understandeach point brought out, for theseprinciples provide the foundationfor an radio and television work

which is to follow.


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nese questions are deaiped to test your knowledge of w. leaoa. ...... a':lr.0. , . nr:-t to see if you can answer them. I f you feel confident that :roathen wr1te out y o u ~ answer., numberinr them to correspond to the qul&dI f you are not.confa::nt that '!ou can answer the questions, re-study the hi iDi lone m ~ r e ~ m e s f ~ r e wrttinr out your answers. Be sure to answer neryquesUon, or 1 you a .to answer a question, it will reduce your grade on thisaes&on When all quesuons have been answered. mail them to us for grading.

QUESTIONSNo. 1. Does a magnetic lield exist about a stationary electron?No. 2. What are the two typa of forcn_ difcuased in ?,tis lesson1

I f the north pole of one mapet is placed Jear the south pole of anothero. a. magnet, how will the mapeta react?No. -&. Do the mapetlc Hnes of force stop at the ends of a mquet. er do tJae,exist in space around the mapet as well as Inside i t !No. 5. If a mapet is suspended 80 that It is free to 8tions wiD the enda of the macnet point7No. 6. I f a bar mar:et is broken in two, will each piece ...,_-.,. a .."._.mapet1No. 8. What wiD bapnetic field?

Atty electron mmoS . ~ ~ h VfDltd bJ' a mapedc 8eld.of the electron I ..... atnnlth of the mapetlc ti ld laereeltor decl'eue!

No. 9.

Ma. 10. U the tip of a aerew dlfMr Ia ....._. C!D a mapet, wiD the . . . . . , drtrtip aa mapetlsedf


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sprayberry academy of radio - nd-3 - lines of force

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