It is done by taking advantage of one of two facts: firstly that the electrical resistance at the junction of two different metals (a thermocouple) varies according to the temperature, and secondly that the volume of a body varies with its temperature. In the first case a constant voltage is applied to the junction of the two metals, and the resulting current is measured with an ammeter calibrated in degrees of temperature; as the temperature at the junction varies, so will the current flowing and therefore the indicating hand on the dial of the instrument. The distance between the thermocouple and the indicator is immaterial, and this form of measurement is very suited to remote temperature reading.