Oscar F. Dame

' First of all, among electrical testing instruments, is the volt-meter, which, as the name implies, serves to measure the voltage or electromotive force of any source of electricity.

In the absence of a complete testing outfit comprising wheatstone bridge, balances and galvanometer no one instrument combines so many advantages, and for the measurement of battery strength and testing the resistance of all sorts of electrical appliances and circuits, the amateur who has become familiar with this instrument, will use it in ordinary work in preference to all others on account of its convenience and time-saving advantages. While not possessing the extreme accuracy of the expensive bridge, the accuracy of the volt-meter is close enough for all practical purposes.

The calculation of resistance with a volt-meter depends solely upon the falling off of the battery voltage when a resistance is connected in series with the test battery and instrument. A simple formula serves to reduce the difference in voltage to ohms, and as the falling off of the battery voltage is in exact proportion to the resitance of the circuit or device being measured and the resistance of the volt-meter, the following method is the simplest:

Calling the full battery voltage E, the voltage through the external resistance, e, the unknown resistance X and the resistance of the volt-meter, r, our formula becomes

X = E X r / e --r or, multiply the voltage of the battery by the resistance of the volt-meter, and divide this result by the second voltage obtained through the resistance under measurement and then subtract from this the resistance of the volt-meter. The result will be the resistance desired, in ohms.

Example. Resistance of the volt-meter, 1,600 ohms (different types of meters vary in this resistance) ; test battery measures 10 volts. With a coil of unknown resistance looped into the circuit in series the meter indicates a reading of 4. What is the resistance of the coil of wire ?

Solution. E = 10, r1,600, and e = 4; therefore (10 X 1,600) divided by 4 = 4,000. Subtract 1,600 from this and 2,400 equals the resistance of the coil in ohms.

By studying this formula closely the amateur will note that when the battery used for testing is constant in voltage, that is, remains right at so many volts without any noticeable variance, the quantity E X r becomes constant also, and the only variable quantity is the unknown resistance, so that by working out a number of examples a table can be compiled by which one may tell, as soon as the volt-meter needle comes to rest, just what the resistance is without further mathematics. We find, therefore, that (E = 10) X (r== 16,000) = 16,000. This never changes if the batteries are kept at standard.