Harold Stephen Black

Harold Stephen Black (1898-1983), American research engineer. He is best known for inventing the negative feedback amplifier, a device that produced clearer telephone signals, especially over long distances.

Born in Leominster, Massachusetts, Black studied engineering at Worcester Polytechnic Institute. After he graduated in 1921, he worked for the engineering department of Western Electric Company from 1921 to 1925. He then worked eight years at Bell Telephone Labs in Murray Hill, New Jersey, and finally as a research scientist and communications consultant for General Precision, Inc., in Little Falls, New Jersey.

When Black joined Western Electric, the telephone industry was seeking a way to increase the strength and clarity of electrical signals used in long-distance telephone systems. The industry used amplifiers to increase the strength of an electric signal. However, telephone lines could need several hundred amplifiers throughout their course to send the signal from one point to another. The result was a buildup of distortion as distance increased, and some difficulty understanding the speaker’s voice.

In 1927 Black developed the mathematical theory of a negative feedback amplifier. His idea was to introduce signals opposite to the ones being relayed down the line. The process would correct many of the small distortions that build up from one amplifier to the next. Black’s theory led to the design of a negative feedback amplifier. His invention made high-fidelity amplification possible in telephone, radio, and telegraph systems.

Black also researched and developed a way to allow a single pair of wires to carry several conversations. He studied and improved telegraph systems, microwave radio relay systems, and ways to code telephone signals for security. He wrote Feedback Amplifiers (1936) and Modulation Theory (1953).