Bell Telephone Laboratories - The Eye That Never Closes
November 1946 Radio-Craft

November 1946 Radio-Craft [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

Here is something you probably didn't know. I certainly didn't, but I'm not surprised. As with many advances in electronics and communications, Bell Telephone Laboratories invented the thermistor. According to this infomercial from a 1946 issue of Radio-Craft magazine, the thermistor was developed to adjust the amplification needed in its relay circuits to provide proper signal strength. It monitored the temperature of the output stage to determine the level. Bell claims the device was sensitive to within one millionth of a degree and could "see" the warmth of a man's body a quarter of a mile away on the battlefield (during World War II). As with most things, improvements have been continually in the thermistor in terms of temperature range, stability, accuracy, ruggedness, etc., all while lowering the price as mass production helps amortize production costs across huge volumes.

Bell Telephone Laboratories - The Eye That Never Closes

You are looking at a thermistor - a speck of metallic oxide imbedded in a glass bead hardly larger than a pin-head and mounted in a vacuum. The thermistor was developed by Bell Telephone Laboratories to keep an eye on the amplification in long-distance telephone circuits.

When a thermistor is heated, its resistance to electric current changes rapidly. That is its secret. Connected in the output of repeater amplifiers, it heats up as power increases, cools as power decreases. This change in temperature alters the resistance, in turn alters the amplification, and so maintains the desired power level. Current through the wire at the left provides a little heat to compensate for local temperature changes.

Wartime need brought a new use for this device which can detect temperature changes of one-millionth of a degree. Bell Laboratories scientists produced a thermistor which could "see" the warmth of a man's body a quarter of a mile away.

Thermistors are made by Western Electric Company, manufacturing branch of the Bell System. Fundamental work on this tiny device still continues as part of the Laboratories program to keep giving America the finest telephone service in the world.

Bell Telephone Laboratories

Exploring and Inventing, Devising and Perfecting for Continued Improvements and Economies in Telephone Service

Posted March 25, 2022