Hot-wire barretter
Early thermoresistive detector for amplitude-modulated signals.
The hot-wire barretter was a type of demodulating detector invented by Reginald Fessenden in 1902. It saw limited use in early radio receivers. Functioning as a highly sensitive thermoresistor, it could demodulate amplitude-modulated signals—a capability the coherer, the standard detector at the time, lacked. As the first device to achieve this, it was eventually replaced by the electrolytic detector, also generally credited to Fessenden.
The device relied on the principle that a metal's resistivity increases with temperature. A direct current bias heated an extremely fine platinum wire to its most sensitive operating temperature. When an oscillating current from the antenna passed through this wire, the wire heated further as the current rose and cooled as it fell. These temperature changes altered the wire's resistance in response to the signal. Because the wire had low thermal mass, it could change resistance quickly enough to follow audio frequencies, but not the much higher radio frequencies. The signal was thus demodulated: the biasing current varied with the changing resistance, and headphones connected in series with the DC circuit rendered these current variations as sound.
The construction, as described in Fessenden's 1902 patent, began with a fine platinum wire about 0.003 inches in diameter embedded in a silver tube roughly 0.1 inches in diameter. This composite wire was drawn until the silver reached about 0.002 inches in diameter, reducing the platinum proportionally to a final diameter of 0.00006 inches—a product known as Wollaston wire. A short piece of this composite wire had its silver cladding etched away, leaving the extremely fine platinum wire. This was supported in a loop on two heavier silver wires inside a glass bulb. The leads passed through the glass envelope, and the entire device was evacuated and sealed.
- Inventor
- Reginald Fessenden
- Year of invention
- 1902
- Platinum wire diameter
- 0.00006 in
- Silver tube diameter
- 0.1 in
- Final silver wire diameter
- 0.002 in
Lore & Background
Fessenden's 1902 patent describes the construction of the device. A fine platinum wire, about 0.003 in in diameter, is embedded in the middle of a silver tube having a diameter of about 0.1 in. This compound wire is then drawn until the silver wire has a diameter of about 0.002 in; as the platinum wire within it is reduced in the same ratio, it is drawn down to a final diameter of 0.00006 in. The result is called Wollaston wire. The silver cladding is etched off a short piece of the composite wire, leaving an extremely fine platinum wire; this is supported, on two heavier silver wires, in a loop inside a glass bulb. The leads are taken out through the glass envelope, and the whole device is put under vacuum and then sealed.
The hot-wire barretter depends upon the increase of a metal resistivity with increasing temperature. The device is biased by a direct current adjusted to heat the wire to its most sensitive temperature. When there is an oscillating current from the antenna through the extremely fine platinum wire loop, the wire is further heated as the current increases and cools as the current decreases again. As the wire heats and cools, it varies its resistance in response to the signals passing through it. 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. However, it cannot vary its resistance fast enough to respond to the much higher radio frequencies. The signal is demodulated because the current supplied by the biasing source varies with the changing wire resistance. Headphones are connected in series with the DC circuit, and the variations in the current are rendered as sound.
Reader's Guide
The hot-wire barretter was the first device used to demodulate amplitude modulated signals, a capability the coherer lacked. Its principle is still used as a detector for microwave radiation, similar to a bolometer. Although it found only limited use in early radio receivers, it represented a significant step in demodulation technology. The barretter was later superseded by the electrolytic detector, also generally attributed to Fessenden. The device's reliance on the thermal inertia of an extremely fine platinum wire—drawn to a diameter of 0.00006 in via the Wollaston wire process—allowed it to respond to audio-frequency variations in signal strength while ignoring the much higher radio frequencies. This thermal response, combined with a DC bias current that heated the wire to its most sensitive temperature, enabled the barretter to convert amplitude-modulated signals into audible sound through headphones. Its legacy endures in modern microwave detection, where the same thermoresistive principle is applied.
Did You Know?
- The hot-wire barretter was invented in 1902 by Reginald Fessenden.
- The platinum wire in the barretter was drawn to a final diameter of 0.00006 in using Wollaston wire.
Frequently Asked Questions
Who invented the hot-wire barretter and when?
Reginald Fessenden created the hot-wire barretter in 1902. It was designed as a demodulating detector for early radio receivers.
What does the hot-wire barretter actually do?
It acts as a highly sensitive thermoresistor capable of demodulating amplitude-modulated signals. This was a capability the coherer, the standard detector of the era, simply could not provide.
How does the hot-wire barretter work?
A direct-current bias heats an extremely fine platinum wire—just 0.00006 inches in diameter—to a high temperature. Because a metal's resistivity rises as it gets hotter, the wire's resistance shifts in step with the incoming signal, enabling demodulation.
Why is the hot-wire barretter considered a milestone in radio history?
It was the first device ever to successfully demodulate amplitude-modulated signals, a capability no earlier detector had achieved. That breakthrough marked a clear step beyond the limitations of the coherer.
What eventually replaced the hot-wire barretter?
The electrolytic detector, also generally credited to Fessenden, superseded it in practical use. The barretter saw only limited adoption in early receivers before being overtaken by this newer design.
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