Signedon face of dial indicator: Cardews Voltmeter / PATENTED JUNE 1 1886 / No 289 / QUEEN & CO. / Phila.
FunctionThis is one of the first models of voltmeters that could be used to measure the voltage of Alternating Current (AC). It can also read the voltage of DC current. Traditional electrostatic gauges cannot be used to measure AC as the voltage and current change direction very rapidly.
The Cardews hot-wire voltmeter does it by measuring the temperature of a long wire that has been connected to the voltage on its ends: For a given electrical resistance in the long wire, the temperature that the wire reaches will be proportional to the voltage to which it is connected (even if it is AC). This temperature alters the length of the wire, which acts on the indicator needle.
See extended comments on the instrument's card.
ProvenanceFrom the Department of Physics, Jefferson Laboratory, Harvard University.