that only a small part of the total current flows through the meter. The dial is then marked to read large values of current, although here again, only a small current is actually passing through the armature coil. Thus it is possible to measure amperes while using a milliammeter with a shunt. External shunts as in Fig. 23, can also be used.
An ammeter must be connected in series with the load. The same readings will result whether the ammeter is placed in the negative connecting wire or in the positive wire--it is a very interesting principle of electricity that the same amount of current flows through any point in a series circuit. The energy used to produce the movement of the needle, and consequently the deflection of the needle, is in proportion to the amount of current flowing through the circuit. Therefore, by first knowing how much current flows, the scale of Fig. 22 can be marked in
Fig. 24
amperes, or milliamperes, then when the same amount flows at some other time, the meter will tell us so by deflecting the needle to the same position.
THE D.C. VOLTMETER
If we take any ammeter with a series resistance load and change the amount of voltage across the circuit, then the needle will deflect in proportion to the change in voltage. Reduce the voltage one-half, then the current through the meter will be on-half. Reduce the voltage still more and the meter needle will deflect less. The amount of voltage used each time can be marked on the scale and is evident that such an arrangement will show when similar voltages are used again. The combined instrument of an ammeter and a comparatively large load in series, as shown in Fig. 24, is known as a voltmeter.
