Better

κ. a . Opiinger 3 Instrument Springs r. W. Carson 9

A Portable Oscillograph

With Unique Features

SIMPLIFIED portable oscillograph having a number of new optical and electrical features is described in this paper. The optical system, which consists of a combination of cylindrical lenses with axes at right angles, is designed to permit simultaneous viewing and photographing. A continuous time axis for both the viewing screen and film is secured by means of a small, variable speed, re-volving mirror.

New galvanometers have been developed which have electro-magnetic damping instead of the usual oil damping. These gal-vanometers are very rugged and have been built for recording fre-quencies as high as 14,000 cycles per sec.

The oscillograph is entirely self-contained and may be operated from a 110-volt 60-cycle lighting circuit without auxiliary attach-ments. The compactness and portability of the instrument can be seen from its overall dimensions which are 8 in. by IIV2 in. by 11 in. and from its total weight, which is approximately 18 lb.

Other features which are claimed for this oscillograph are an optical system which magnifies the galvanometer deflections and gives a brilliant trace that can be observed in a brilliantly lighted room; a simple method of taking photographs similar to that in an ordinary camera; a large viewing screen for making tracings or for giving demonstrations to a group of persons; and simplicity, compactness, and ruggedness comparable with the average electrical indicating instrument. Although this oscillograph is a research development product, it will be made available in the near future in a form that will differ only in minor details from the instrument described. (A.I.E.E. paper No. 33-90)

Classification of Bridge Methods

of Measuring Impedance J . o . rergusorr

ELECTRICAL measuring instruments play an impor-tant part in the generation, distribution, and sale of electrical power, and in the development and testing of electrical machinery. The accuracy of electrical measuring instruments depends as much upon the quality of the control springs as on the design of the torque pro-ducing elements.

Unstable effects found in the application of spiral springs to elec-trical instruments arise from aging in service and hereditary hys-teresis in the spring material. There is little available information in the technical literature on these effects. In order to produce the most satisfactory spring controlled instruments for the electrical industry, there should be recorded in the technical literature a con-siderable body of detailed information on springs. Information is needed on such subjects as the effect of composition, condition of ma-terial, forming methods, stabilizing treatments, design details, resid-ual stresses, service conditions, and temperature on the performance of spiral instrument springs.

The information presented in this paper resulted from torsional pendulum tests, hardness tests, spring uncoiling tests, various form-ing and stabilizing treatments, and measurements of hereditary hysteresis with the "grid-glow micrometer."

The results of these tests are presented graphically, showing that moderate temperature heat treatment has an important effect on hereditary hysteresis, that the softening range of cold worked spring ribbon is very critical, that the forming temperature and forming time have a controlling effect on residual stresses, that a stress relief anneal reduces aging and hereditary hysteresis. The nature of the action of hereditary hysteresis is defined.

Additional information is needed on the effect of composition, mechanical condition of the spring material, rolling practice, and the temperature of loading on hereditary hysteresis. Also, the bib-liography may not be complete. Discussion on these subjects is invited by the author. (A.I.E.E. paper No. 33-89)

K̂N ANALYSIS of the requirements for satisfactory operation of the simple 4-arm bridge when used for impedance meas-urements is given in this paper. The various forms of bridge are classified into 2 major types called the ratio-arm type and the prod-uct-arm type, based on the location of the fixed impedance arms in the bridge. These 2 types are subdivided further, based upon the phase relation which exists between the fixed arm impedances. Eight practical forms of bridges are given, 3 of them being duplicate forms from the standpoint of the method of measuring impedance. These bridges together allow the measurement of any type of im-pedance in terms of practically any type of adjustable standard. The use of partial substitution methods and of resonance methods with these bridges is discussed and several methods of operation are described which show their flexibility in the measurement of im-pedance.

The article furnishes a comparison of the relative merits of the large number of circuits which are available for making the same measurement and should serve as a guide to the engineer who is more interested in results than in acquiring a broad education in bridge measurements. An outline is given of the fundamental requirements which must be met by bridges used for impedance measurements, and a classification is made which serves as a help in the choice of a bridge for any particular type of measurement. The relative merits of the simpler types of bridge are discussed from the standpoint of the measurement of both components of an impedance, particularly with reference to measurements in the communication range of frequencies from about 100 to 1,000,000 cycles. Where only the major com-ponent of an impedance is desired, for instance where only the in-ductance of a coil or the capacitance of a condenser is desired, the requirements are not so severe and many forms of bridges may be used which are not suitable for the purpose here outlined. Bridges are also used to a large extent for other purposes than impedance measurements, such as for frequency measurements; these applica-tions are not considered. (A.I.E.E. paper No. 33-88)

8. Bell Telephone Laboratories, Inc., New York, Ν. Y. 9. McGraw-Hill Publishing Company, Inc., New York, Ν. Y., formerly with the Westinghouse Electric and Manufacturing Company, Newark, N. J.

The Expulsion Κ. B. McEachron 1 1

n . ~ I. W. Gross 1 2

Protective oap h. l. Melvin 1 3

THE EXPULSION protective gap consists of a fiber tube of proper bore and dimensions so arranged with an internal gap that lightning discharges are conducted within the tube and the power follow current is stopped through the expulsion action of the tube. Its action during interruption of follow current is similar to that of the expulsion fuse. In practice the expulsion gap is provided with a series gap, the whole assembly being electrically in parallel with the insulation to be protected.

On steel tower lines the application is limited to grounded neutral circuits. However, with wood pole lines where the wood insulation of the pole can be utilized, protection of circuits with ungrounded neutral can probably be realized.

Expulsion protective gaps have a maximum and minimum current rating. This means that for proper functioning the device should be chosen so that the phase to phase short circuit current must not ex-ceed the maximum rating while the minimum current to ground should not be less than the minimum rating of the expulsion gap.

Field tests were made on expulsion gaps on the 132-kv system of the Appalachian Electric Power Company at Glen Lyn, Va. Crest currents as large as 6,700 amp were interrupted successfully in V2 cycle. One tube interrupted system short circuit current 11 times without showing signs of serious erosion, which indicates that tube life will probably not be limited by the effect of electrical dis-charges. The tests were entirely successful and indicate that on 132-kv lines expulsion gaps deserve serious consideration as a prac-tical device in rendering a transmission line lightning proof.

11. General Electric Company, Pittsfield, Mass. 12. American Gas and Electric Company, New York, Ν. Y. 13. Electric Bond and Share Company, New York, Ν. Y.

414 ELECTRICAL ENGINEERING