Hot-wire barretter

The hot-wire barretter was a demodulating detector, invented in 1902 by Reginald Fessenden, that found limited use in early radio receivers.

In effect, it was a highly sensitive thermoresistor, which could demodulate amplitude-modulated signals, something that the coherer (the standard detector of the time) could not do.

[1] The first device used to demodulate amplitude modulated signals, it was later superseded by the electrolytic detector, also generally attributed to Fessenden.

Because of the low thermal mass of the wire, it is capable of responding quickly enough to vary its resistance in response to audio signals.

The signal is demodulated because the current supplied by the biasing source varies with the changing wire resistance.