Hot-wire barretter

Lore & Background

The hot-wire barretter was constructed using a fine platinum wire, about 0.003 inches in diameter, embedded in a silver tube. This composite wire was drawn down until the silver reached 0.002 inches in diameter, reducing the platinum to 0.00006 inches. The silver cladding was etched off a short piece, leaving an extremely fine platinum wire supported on two heavier silver wires in a loop inside a glass bulb, which was evacuated and sealed.

Reader's Guide

The hot-wire barretter operated on the principle that a metal's resistivity increases with temperature. A direct current biased the device, heating the fine platinum wire to its most sensitive temperature. Oscillating current from the antenna further heated the wire as current increased and cooled it as current decreased, varying its resistance in response to the signals. Because of its low thermal mass, the wire could respond quickly enough to audio signals but not to higher radio frequencies, thus demodulating the signal. Headphones in series with the DC circuit rendered the current variations as sound. The barretter principle is still used as a detector for microwave radiation, similar to a bolometer.

Did You Know?

• The hot-wire barretter was the first device used to demodulate amplitude-modulated signals.
• It was invented in 1902 by Reginald Fessenden.
• The device used an extremely fine platinum wire, drawn to a diameter of 0.00006 inches (1.5 μm).
• The barretter principle is still used as a detector for microwave radiation, similar to a bolometer.