Crystal Fire #2

A long strand of aluminum or copper wire tacked to the roof of a house or strung between trees served as an antenna to capture radio signals. Electrons in the wire oscillated back and forth as these waves passed, like corks bobbing on water. Another wire coiled around a cylinder provided a tuning device to select the specific radio frequency transmitted by a station and to eliminate unwanted signals. And a pair of earphones translated the tiny pulses of electric current back into the words or sounds that had been spoken or played into a microphone at the station.

A crystal detector converted the back-and-forth alternating current in the antenna and tuning circuit into one-way bursts of direct current required by the earphones. Exactly how crystals worked had been a mystery until the 1920s, despite their having been used. In 1874 Ferdinand Braun discovered that currents in crystals flowed more readily in one direction, and with a sharp wire tip pressed into a crystal face flowed in a single direction. This is called rectification.

Hearing about Marconi’s difficulties in sending signals long distances relying on a spark between two electrodes starting in the mid-1890s, Braun developed a new kind of sparkless transmitter, which eventually allowed transmitting voice and music, not mere Morse code.

Chapter 2 of Crystal Fire also gives personality and biographical sketches of Brattain, Bardeen, and Shockley.