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Page:Popular Science Monthly Volume 90.djvu/968

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��Popular Science Monthly

��the connections in first diagrams. The ammeter will read correctly, but the volt- meter records the voltage-drop across both.

This arrangement is very much in error because the drop in the ammeter is greater than in the resistance. In problem 2 the voltmeter is connected across the resistance only, as in second diagram. The current flowing through the voltmeter will be very small compared to that flowing in the resistance. The per cent of error in the ammeter is small. The voltmeter in the connections shown will read correctly.

A simple problem may serve to make clear the described method. What is the correct resistance of a 40- watt lamp? The ammeter is a milliammeter, resistance of .045 ohm. The voltmeter has a resistance of 15,000 ohms. The voltmeter reads no volts and the ammeter .3636 amp.


Problem i. Voltmeter Across Resis- tance.

The current through voltmeter, by Ohm's Law, equals no

= .00734 ampere.


The current through resistance equals •3636 — .00734 = .35626 ampere.

Per cent of error, .0073

= 2.43 per cent.


The voltmeter records correctly.

Problem 2. Voltmeter Across Resistance AND Ammeter. Voltage across ammeter, .3636— .045 = .1638 volt.

Voltage across resistance,

iio.oooo — .1638 = 109.8362 volts.

Per cent of error, .1638

��.149 per cent. 109.8362

The ammeter records correctly.

The ammeter in problem i reads too high by 2.43%, or about 1.51 ohms. From the arrangement in problem 2 the voltage reads too high by only .149%, or an equivalent of .0595 ohms.

From the foregoing method a rule may be summed up as follows : When measuring a high resistance, connect voltmeter across resistance and ammeter both ; when meas- uring a low resistance, use voltmeter across resistance only. — T, E. Martin.

���Increasing the Efficiency of the Pouisen Arc Generator

IN THE past the Pouisen arc generator of undamped or sustained waves has usually been used directly in series with the antenna in which it operates. The effi- ciency of this arrangement is variously stated as from 15 per cent to 50 per cent, depending largely upon the constants of the circuits and the details of design of the arc generator itself. The wide use to which generators of this type have been put in the past few years, in the United States Navy

and elsewhere, has caused a good deal of study to be put upon the sys- tem with a view toward main- taining the highest effi- ciency. By the addition of condenser and inductance cir- cuits it has been found possible to in- crease materi- ally the output of a single arc oscillator, and some results have been secured which would appear most surprising to those who are familiar with only the older forms of the apparatus. United States patent number 1,179,353, issued in 1916 to L. F. Fuller, shows one method of increasing the radiation or antenna current produced by an arc generator. As indicated in the figure, the only material change is that of adding the two condensers i and 2. The first of these is placed directly in series with the arc 3 (which is supplied with power through the leads 4 and 5 in the usual way), and the second is shunted about the arc and the first condenser. Above the arc and shunt circuit is connected the loading inductance 6, with a small portion arranged to be short- circuited by means of the signaling key; the high potential end of this coil is led directly to the aerial 7. The capacity of the two condensers is adjusted by trial, and depends upon the size of the antenna and the power of the arc. When the best values are used, the antenna current is often more than doubled without using any more power than normally.

��New arc circuit which increases the radiation

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