How did electrical feedback improve long-distance telephone amplifiers in the 1930s?

In the 1930s, long-distance telephone amplification was significantly improved by the development of the negative-feedback principle, pioneered by American electrical engineer Harold Stephen Black.

While vacuum tube amplifiers had previously enabled transcontinental communication, they suffered from significant distortion that hindered signal quality over long distances. Black addressed this by devising a system where a portion of the amplification output is fed back into the input in a negative phase. This process effectively cancels out the distortion generated within the amplifier, resulting in steady, high-fidelity signal transmission.

This breakthrough was essential for the reliability of long-distance telephone networks and later became a foundational concept for radar, servomechanisms, and various other control systems.1