Question

Hotwire ammeters are used for measuring _____________ currents.

• A. A.C. only
• B. D.C only
• C. both A.C. and D.C
• D. none of these

Answer 1

The correct option is C both A.C. and D.C

Hot wire instruments use the Joule’s law of thermal expansion. When a current (AC or DC) is passed through a special wire, the wire heats up due to $\left(I^2\right)\times R$ loss. This heating increases the length of the wire. This increase is recorded as the current. The calibrations indicates an aggregate value which goes with both AC and DC measurements. Over usage can tear the wire apart and render it un-usable. These also measure true RMS for an applied AC.

Hence they are used for measuring both AC and DC.