A hot-wire movement is a historical movement in which the expansion of a wire provides the pointer deflection. The hot-wire movement consists of a hot wire tensioned by a coil spring, to which the instrument pointer is attached via a small roller.
flows through the hot-wire, it expands and the coil spring moves the instrument pointer. Themore current flows through the hot-wire, the greater its expansion and the associated pointer deflection.
The expansion of the hot-wire depends on the power that has been dropped across the hot-wire, regardless of whether it is alternating or direct current. Since the expansion of the hot-wire depends on the power or the current flowing through it as Iexp2xR, there is a quadratic relationship between the expansion and the rms current, which is indicated by the non-linear instrument scale.