Thermal heating

Another phenomenon, sometimes useful in the measurement of electric currents, is the fact that whenever current flows through a conductor having any resistance, that conductor is heated. All conductors have some resistance; none are perfect. The extent of this heating is proportional to the amount of current being carried by the wire.
By choosing just the right metal or alloy, and by making the wire a certain length and diameter, and by employing a sensitive thermometer, and by putting the entire assembly inside a thermally insulating package, a hot-wire meter can be made. The hot-wire meter can measure ac as well as dc, because the current-heating phenomenon does not depend on the direction of current flow.
A variation of the hot-wire principle can be used by placing two different metals into contact with each other. If the right metals are chosen, the junction will heat up when a current flows through it. This is called the thermocouple principle. As with the hot-wire meter, a thermometer can be used to measure the extent of the heating.
But there is also another effect. A thermocouple, when it gets warm, generates a direct current. This current can be measured by a more conventional, dc type meter. This method is useful when it is necessary to have a faster meter response time. The hot-wire and thermocouple effects are used occasionally to measure current at radio frequencies, in the range of hundreds of kilohertz up to tens of gigahertz.