Thermal  heating

Another phenomenon, useful in measurement of electric currents, is the fact that whenever current flows through the conductor having any resistance, that conductor is heated. All conductors posses resistance; none are ideal. The extent of this heating is proportional to amount of current being carried by wire.

By choosing the right metal or alloy, and by making the wire a specific length and diameter, and by using a sensitive thermometer, and by putting the entire assembly inside a insulating package, a hot-wire meter can be made. The hot-wire meter can measure alternating current as well as direct current, as the current-heating phenomenon does not depend on direction of current flow.

A variation of hot-wire principle can be used by placing 2 different metals into contact. If the right metals are opted, the junction will heat up when a current flows through it. This is known as thermocouple principle. As with the hot-wire meter, a thermometer can be used to measure the heating.

But there is another effect. A thermocouple, when gets warm, generates a direct current. This current can be measured by straight, direct current type meter. This method is useful when it is required to have a faster meter response time. The both hot-wire and thermocouple effects are used infrequently to measure current at radio frequencies, in range of hundreds of KHz up to tens of gigahertz.