[Antennas] RF Ammeters

[Adam Farson]

Hi Ron,

The thermocouple and hot-wire ammeter work on completely different
principles. In the thermocouple ammeter, current passing through a heater
element thermally bonded to a thermocouple heats a junction of two
dissimilar metals. This develops a DC voltage which deflects a moving-coil
microammeter.

In the hot-wire instrument, current passing through a wire made of nichrome
or a similar alloy causes the wire to expand longitudinally (extend). A
lever or gear mechanism mechanically amplifies this small motion, deflecting
the pointer.

The hot-wire ammeter is so delicate, and has such poor accuracy and
repeatability, that the thermocouple ammeter supplanted it well before WW2.

Cheers for now, 73,
Adam VA7OJ/AB4OJ