Hotwire ammeters are used for measuring _____________ currents.

Hotwire ammeters are used for measuring both AC and DC  currents.

Joule's law of thermal expansion uses hot wire instruments. A specific wire warms up when a current (AC or DC) is carried through it due to $I^{2}R$ loss. The wire's lengthening is caused by this heating. The current is defined as the rise in value. The calibrations show a total value that applies to both AC and DC readings. The wire may become unusable if it is overused and torn apart. These measure the actual RMS of an applied AC.
As a result, they can measure both AC and DC.