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FRANK HERTZ EXPERIMENTModel: FH-3001

Manufactured by:SES lnstruments Pvt" Ltd.452, Adarsih Nagar,Roork ee-247 667 U KPh 01 332-?.72852, Fax ' 277 1 18Email : info@sesteqh no. com

sestech n io. india@g mail. comWebsite. www. sestech no. com

G@,lso 9001ISO 9001: 2008 CERTIFIED

INTRODUCTION

From the early spectroscopic work it is clear that atoms emitted radiation atdiscrete frequencies; from Bohr's model, the frequency of the radiation v is related to the:hange of energy levels through AE:hv. It is then to be expected that transfer of energy toatomic electrons by any mechanism should always be in discrete amounts. One suchnlechanism of energy transfer is through inelastic scattering of low-energy electrons.

Frank andHertz in r9r4 set out to verify these considerations.

OPERATING PRINCIPLE

The Frank-Hertz tube in thisinstrument is a tetrode filled withthe vapour of the experimentalsubstance Fig. 1 indicates the basicscheme of experiment.

The electrons emitted byfilament can be accelerated by thepotential Vczr between the cathodeand the grid Gz. The grid Gr helpsin minimising space charge effects.The grids are wire mesh and allcwthe electrons to pass through. Theplate A is maintained at a potentialslightly negative with respect to thegrid Gz. This helps in making thedips in the plate current moreprominent. In this experiment, theelectron current is measured as aftinction of the voltage Vc2r. As the

electron bombardment.

atoms always had discrete values.levels were in agreement with

vczn

vczrc

(i) It is possible to excite atoms by low energy(ii) The energy transferred from electrons to the(iii) The values so obtained for the energy

spectroscopic results.Thus the existence of atomic energy levels put forward by Bohr can be proveddirectly' It is a very important experiment and can be performed in any college orUniversify level lab.

srvitched

ticularly

Fig Ivoltage increases, the electron energy goes up and so the electron can overcome theretarding potential V6ra to reach the plate A. This gives rise to a current in the ammeter,which initially increases. As the voltage further increases, the electron energy reaches thethreshold value to excite the atom in its first allowed excited state. In Ooing so, theelectrons lose energy and therefore the number of electrons reaching the plate d"..r.ur.r.This decrease is proportional to the number of inelastic collisions that have occurred.When the V6r6 is increased further and reaches a value twice that of the first excitation

Nanoammeter

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+,goLLJoE(EfcoL+,o(t,EI

1400

1200

1 000

800

600

400

2AA

0

Fig. 3 Plot of Beam CurrentVs. Accelarting Voltagein Frank HerE b(Periment, FH'300{

30 40 50 60 70Acce larating Voltage V*^(V)

Fig. 4 Oscilloscope display of Frank-Hertz Experiment

potential, it is possible for anelectron to excite an atom halfwayt,e nveen the grids, loose all itscnerg)', and then gain anew enough Vczx::l:rsv to excite a second dip in the: -:rrenr. The advantage of this flpe

: f configuration of the potential is:rat the current dips are much moreironounced, and it is easy to obtain:li'e fold or even larger multiplicity:n the excitation of the first level.

Frank-Hertz ExperimentSet-up, Model : FH-3001, consistsof the following :

Fig. 2: Configuration of the potentialin Frank-He rtz Arrangement

. Argon filled tetroder Filament power supply :2.6 - 3.4v continuousry variabrer Power Supply for V6,K : i.3 _ 5V continuously variabler Power Supply for V6ra : 1.3 _ l2V continuously variablee Power Supply for V6r6 : 0 _ 95V continuously variable. Saw tooth waveform for CRO display

Scanning Voltage : 0 _ 95ltScanningFrequency : il5 t20Hz.

. Multirange Digital VoltmeterRange : 0 _ I00V,with 10096 overDisplay: 3 % digit 7-segment LED with auto polarity and decimal indication

o Multirange Digital AmmeterRange :0_ 100,0-10 pA &0_tpADisplay:3 % digit 7_segment LED with auto polarity

The instrument can not only lead to a plot of the amplitude spectrum curve bymeans of point by point measurement, but also directly display the amplitude spectrumcurve on the oscilloscope screen. This instrument can thus be used as a classroomexperiment as well as for demonstration to a group of students.

ANALYSIS OF THE DATA

Data obtained for the excitation potential point by point are shown in Fig. 3. Thereading are taken for lV changes on grid 2 (Vor*). A significant decrease in electron(collector) current is noticed every time the potential on grid 2 is increased byapproximately lZY, thereby indicating that eneigy is transferred from the beam in(bundles) quanta of 12 eV only. Indeed, a promineniiine in the spectrum of argon exists at1048A corresponding to eV=l1.83.

The location of the peaks is indicated in Fig. 3. Average value of spacing betweenpeaks is I 1.75 ev compared with the accepted varui of t t.s:vl

(bundles) quanta of 12 eV only. Indeed, a prominent line in the spectrum of argon existsat 1048,{ corresponding to eV=l 1.83.

The location of the peaks is indicated in Fig. 3. Average value of spacingbetween peaks is 11.75 ev compared with the accepted value of 1l.g3v.

UNPACKING

Unpack the instrument carefully and check the accessories with the packing list.The instrument is checked thoroughly before dispatch, damage/shortage,li any shouldbe reported immediately.

Take out the Frank-Hertz Tube from its window-marked 'Frank-Hertz TubeWindow' byremoving its cover.

n 12 3 13 14 g

PANEL CONTROLS AND THEIR FUNCTIONS1) Ammeter2) Voltmeter3) Manual - Auto Switch4) Scanning Voltage Knob5) Filament Voltage Knob6) Current Multiplier Knob7) Voltage Display Selector: Vc,x,

Vnro or V6r6VG16 Adjust knob :Vnro Adjust Knob :Vcr^ Adjust knob :Power SwitchY-Output TerminalGround TerminalX-output TerminalPower LeadFrank Hertz Tube

8)e)10)11)12)13)14)15)16)17)

1.3 - 5V1.3 - 15 V0-80v

Frank-Hertz Tube Window

INSTALLATION

Before the Frank-Hertz tube is put in its socket, make sure the power supplies-'Vo,*, Yczo & V6rp are working properly. For this proceed as follows.1. Put all the control knobs (scanning voltage v6,K, Filament voltage vcze &

Accelerating voltage v6rK Knobs) to their minimum position by rotatinganticlockwise.

2. Turn the Manual-Auto switch to Manual3. Tum Voltage Display Selector to V6,s and rotate the V6,6 knob clockwise to see if the

power supply is working properly. Similarly tum the Voltage Display Selector to V6raand V6rK and check if these power supplies are also O.K.

4. Switch 'OFF' the power and put Frank-Hertz tube in the socket. As the tube isdelicate and very expensive this operation must be handled very carefulty and bya technical hand only.

The instrument is now ready for operation

OPERATIN G INSTRUCTIONS

1) Before the power is switched 'ON' make sure all the control knobs are at theirminimum position and Current Multiplier knob at 10-7 position.Switch 'ON' the power.Turn the Manual-Auto switch to Manual, and check that the Scanning voltageKnob is at its minimum position.Tum Voltage Display Selector to V6,s and adjust the v6,r knob until voltmeterreads 1.5V.

5) Turn voltage Display Selector to v6ra and adjust the v6ra knob until thevoltmeter read 7 .5.

When you have finished step 1-5, you are ready to do the experiment withfollowing parameters.

2)3)

4)

Filament VoltageVe

,x"1Ve/v" r^Current Multiplier

These are suggestedwith other values also.

mid position1,5 V7.5 VOV10-7 A

values for the experiment. The experiment can be done

6) Rotate V6r6 knob and observe the variation of plate current with the increase ofV6r6. The current reading would show maxima and minima periodically. Themagnitude of maxima could be adjusted suitably by adjusting the filament voltageand the value of current Multiplier. Now take the systematic readings, v6r6 vs.plate current. For better resolution, the reading may be taken at a interval of lV.

Plot the graph with output current on Y-axis and accelerating voltage V6r6 at X-axis.

7) Tum the Manual-Auto switch to 'Auto', connect the instruments y, G, X socketsto Y, G X of oscilloscope. Put the Scanning Range switch of oscilloscope to X-ymode/external 'X'' Switch on the power of oscill,oscope, adjust the y and X shiftto make the scan base line on the bottcim of screen. Rotate the .Scanning Knob, ofthe instrument and observe the wave-form on the oscilloscope screen. Adjust the'Y-gain' and 'X-gain' of oscilloscope to make wave-form ciear and y amplitudemoderate' Rotate the scanning potentiometer clockwise. to end. Then themaximum scan voltage is 85V. Measure the horizontal distance between thepeaks' The distance of two consecutive peaks (count the grids) and multiply itbyY/grid factor (X-gain) of oscilloscope. This would give the vaiue of argon atom,sfirst excitation potential in eV.

PREGAUTION

1) Before taking the systematic readings, gradually increase the value of V621 to amaximum' Adjust the filament voltage ifrequirei such that max. readings is about1000 on Xl0-8 range. This will insure that all the readings could be taken in thesame range.

2) During the experiment (manual),. when the voltage is over 60v, pleas e payattention to the output current indicator, If the ammeter reading increasessuddenly, decrease the voltage at once to avoid the damage of the fube.

3) Whenever the Filament Voltage is changed, please allow 2/3 minutes for itsstabilisation.

4) When the Frank-Hertz Tube is already in the socket, please make sure thefollowing before the power is switched ioN' o. ,oFF,, to avoid damage to thetube.a) Manual -. Auto switch is on Manual and Scanning and Filament Voltageknob u.t 1t. minimum position (rotate it anticl,ockwise) and CurrentMultiplier knob at l0-7.b) Vc,r, VcrR, and V6rK all the three knobs are at their minirnum position.

PACKING LIST

l) Frank-Hertz Main Unit2) Frank-Hertz Tube3) Set of wires for connecting Frank-Hertz Main unit to oscilloscope4) Operating Instruction Manual

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