478SCHOOL SCIENCE AND MATHEMATICS

A real note of appreciation is expressed to John L. Sharpe, Chicago;Allan L. Carl, Chicago; 0. C. Durham, Waukegan; William Einbecker,Highland Park; George Reynolds, Winnetka; James L. Russell, Cleveland;Frances H. McCauley, Warren; Clarence Smith, Aurora; John W. Swen-son, Evanston; Charles Hetzler, David Hamlin and E. P. Martz, of Dear-born Observatory Staff, and others whose names were not registered, forbringing their telescopes and giving instruction at the Evanston Com-munity Stargazing Party.

USE THE ELECTRIC OUTLET FORDEMONSTRATING VACUUM TUBES

ANDREW DOUGLASMurphy High School, Mobile, Alabama

A, B, and C "batteries" are often mentioned when discussingelectronic devices. In reality, voltage from any source will causethe desired operations. The majority of circuits used in indus-trial and domestic applications employ no electro-chemical gen-erators. Classroom demonstrations, likewise, can be given usingenergy from the electric supply of the building, and without anyspecial "battery substitute" or "battery eliminator."

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SVVV\AAAA/^

FIG i

Figure 1 shows a way of obtaining several voltages for a triodefrom a single source. The resistance necessary to limit the fila-ment current is divided into two parts. The maximum value ofthe grid negative bias equals the IR drop in J^i. The maximumvalue of the plate voltage equals the IR drop in R^ Sliding con-tacts on these resistances vary Eg and Ep.

DEMONSTRATING VACUUM TUBES 479

This circuit may be used on alternating current. Since theplate current is a stream of electrons whose source is the fila-ment, the plate receives them only when it is positive with re-spect to the filament.Though the plate is actually negative half of the time, it is

cold and refuses to yield electrons. Hence a unidirectional cur-rent flows through the plate ammeter; and any commercial fre-quency causes this to happen so often as to produce a steady in-dication on any ordinary instrument. The grid being negativewhen theplate is positive, its bias then is a voltage whose instan-taneous direction is suitable to repel and control the filamentemission. That is, rapid and periodic reversal of the supply volt-age contributes no ill effect, so far as a demonstration of ele-

FIG. 2

mentary performance is concerned. Therefore, when a’c happensto be the sole available supply (as in many school rooms) thefundamentals of tube behavior can still be considered on a de-scriptive basis.

OTHER ARRANGEMENTSAs shown in Figure 2 the voltmeter between filament and grid

may be taken out. The grid lead is then connected to a slidermaking contact on some sort of voltage divider between thesupply wires. If a uniform slide-wire is used, distances L\ and Lzmeasure the voltage value applied to the grid with reference tothe fixed position of the filament. For example, if ^==|^, fila-ment and grid will be at the same potential (no bias). Movement

480 SCHOOL SCIENCE AND MATHEMATICS

of the slider to the left imposes a negative bias with reduction ofplate current. Movement to the right produces a positive biaswith rise of plate current. A small shift of grid voltage can beshown to bring about a relatively large change of plate current,making plain the amplification effect of the tube. The supplymay be either a-c or d-c.

Figure 3 indicates how a cathode-type tube may be employed.The heater is supplied by any convenient source, the grid is con-nected to one line, the plate to the other, and the cathode to aslider on a potentiometer. The grid can thus be made more nega-tive than the cathode, with results similar to those obtainedfrom the circuit of Figure 1. As with that circuit, only negative

FIG. 3

bias for the grid is possible. To obtain both positive and negativebias, the grid must be connected to the slider of another across-the-line potentiometer. If this is done, complete control of gridand plate voltage is possible.

It must be remembered that these circuits operate on a-c be-cause of self-rectification in the tubes. Cyclic variation of thesupply causes a tube to operate dynamically instead of it beingheld at some one point of its performance. Therefore representa-tive characteristic curves are not obtained from the readings ofr.m.s. or average-value instruments. Qualitative results, how-ever, are acceptable and a demonstration of tube operation ispossible which otherwise might have to be glossed over.

DEMONSTRATING VACUUM TUBES 481

SOME PRACTICAL CONSIDERATIONSIt is well to select a triode requiring low filament current. This

will reduce the size and cost of resistors due to the low wattageto be dissipated. A type 30 is usually convenient. For this,

Ri= 200 ohms, 10 watts.� ^fl800 ohms, 10 watts, (for 120v. supply)

2 \1600 ohms, 10 watts, (for llOv. supply)

A type 27 is suitable where a cathode tube is desired. Alsosuitable is a 25L6 connected as a triode. This latter gives a largerplate current where a sensitive galvanometer is not available forthe plate circuit.

. Any demonstration of this sort should be given after placingwiring, tube, and controls on a vertical board at least 2 ft. X 3ft. All wires should run vertically or horizontally, all should bewidely separated, and all should be as straight between connec-tion points as is possible. The simpler the connections the morecertain it is that principles rather than details are being con-sidered.Acknowledgement should be made of the help given by Mr.

Lloyd E. Smith of the TVA CommunicationsLaboratory in work-ing out the details of Circuit No. 3 and for other helpful sugges-tions.

TALCUM POWDER HELD RESPONSIBLEFOR SERIOUS ILLNESS AFTER

OPERATIONS

Serious complications and illness may follow operations if talcum powderfrom the surgeon^s gloves gets into the surgical wound. A warning of thisdanger is given by Dr. Edward J. McCormick and Dr. Thomas L. Ramsey,of Toledo, Ohio, in a report in the Journal of the American Medical As-sociation.They report two cases of postoperative complications in which second

operations were required, one of them resulting in the loss of the child-bearing organs in a 21-year-old woman.In both cases crystals of magnesium silicate, or talc, were found in the

inflamed tissues removed at the second operations. It is probable, theToledo surgeons believe, that many other cases of postoperative complica-tions caused by talcum or by lycopodium powder have gone undiagnosed inthe past.

Careful washing of the gloved hands of the surgeon and his assistants,to remove the powder used on the gloves, and care to keep the air of theoperating room free of the powder are urged to prevent such complications.