Hot Wire Ammeter (Thermal Ammeter)
Most ammeters rely on the direction of the current, so they read zero on AC. A hot-wire ammeter sidesteps that entirely by measuring heat instead, and this simulation shows the wire expanding, the fine thread taking up the slack, and the pointer swinging across the scale.
How to use this simulation
- Vary the current through the wire
- Switch between direct and alternating current
- Change the resistance of the hot wire
What to look for
- The pointer deflects the same way whichever direction the current flows
- Divisions are crowded at the low end and spread out higher up, because deflection follows I²
- An AC supply gives the reading corresponding to its RMS value, not its peak
The physics behind it
The wire dissipates power at a rate I²R, so the heating — and hence the thermal expansion — depends on the square of the current and not on its direction. That makes the instrument respond to the root-mean-square value of an alternating current, and it reads DC on the same scale. The squared dependence is why the scale is non-linear.