Definition
An electrical current-measuring instrument that works by passing the current through a fine resistance wire, which heats up and expands in proportion to the current flowing through it. The expansion of the wire is mechanically linked to a pointer, giving a reading of current. Because heating depends on the square of the current and is independent of direction, a hot-wire ammeter reads true RMS current and works equally well for direct current (DC), alternating current (AC), and radio-frequency (RF) current.
Plain English
A meter that measures electric current by letting the current heat a thin wire. The hotter the wire gets, the more it stretches, and that stretch moves a needle on a scale. It works for any kind of current, including the high-frequency current going to a radio antenna.
Context Anchor
Seen in discussions of older aircraft electrical instruments and basic electrical measuring equipment.
Derivation
Named directly for how it works: a 'hot wire' that heats up under current, driving the 'ammeter' (ampere meter) movement. Knowing the mechanism is the whole point of the name -- the heat does the measuring.
Why Pilots Care
Provides reliable current readings in vibrating or magnetically noisy aircraft environments where conventional ammeters can give false indications.
Analogy
It is like a thin metal strip that changes shape when warmed. In the ammeter, the electrical current creates the warmth, and the small change in the wire is used to move the pointer.
Intuition Check
“Hot-wire” does not mean bypassing or illegally starting something here. It means the instrument uses a wire that becomes warm when electrical current flows through it.
Example Sentence 1
The technician used a hot-wire ammeter to check that RF current was reaching the HF antenna during the transmitter test.
Example Sentence 2
In flight the hot-wire ammeter helped the pilot detect an excessive electrical load before it drained the battery.