In certain forms, hot-wire instruments are well adapted for the measurement of very small alternating currents.
From this it follows that hot-wire ammeters are generally not capable of giving visible indications below a certain minimum current for each instrument.
In constructing a hot-wire instrument for the measurement of high frequency currents it is necessary to make the working wire of a number of fine wires placed in parallel and slightly separated from one another, and to rpass the whole of the current to be measured through this strand.
In the Hartmann and Braun form of hot-wire voltmeter, the fine wire is fixed between two supports, and the expansion produced when a current is passed through it causes the wire to sag down, the sag being multiplied by a gear and made to move an indicating needle over a scale.
Another form of hot-wire ammeter is a modification of the electric thermometer originally invented by Sir W.