The patent. Greenleaf Whittier Pickard filed on 30 August 1906 and was granted US Patent 836,531 on 20 November 1906, for a means of receiving intelligence communicated by electric waves. The claim covers a detector using a crystal of a non-metallic element — silicon in particular — contacted by a spring-loaded metal point mounted so a sensitive spot can be found. Pickard reportedly tested tens of thousands of mineral and contact combinations to arrive at it.

What is physically happening. The point contact forms what we now call a Schottky barrier: a metal-semiconductor junction whose energy barrier lets carriers cross far more readily in one direction than the other. That asymmetry rectifies the radio-frequency signal, removing one half of the wave so what remains has a non-zero average that follows the audio envelope. None of this was understood in 1906 — band theory did not exist. Pickard had a device that demonstrably worked and no theory to explain it, which is why the search was empirical on such a scale.

Why sensitive spots exist. A natural crystal surface is not uniform. Impurities, grain boundaries and local doping mean rectification is good at some points and absent at others, and the effective junction is microscopic. Hunting for the spot with a whisker is, in effect, probing the semiconductor for a usable junction by hand. When Bell Labs returned to point-contact rectification in the 1940s, that work led directly to the point-contact transistor of 1947 — the crystal detector is its lineal ancestor, not merely an analogue.

Why it died, and why it survived. Vacuum tubes amplified and stayed adjusted, and the fragile whisker could not compete after about 1920. But a crystal set needs no power at all, so they were built from scrap by soldiers in both World Wars — foxhole radios, using a pencil lead or a rusty razor blade as the crystal. That is worth trying here: a blued razor blade and a pencil tip really will detect a broadcast station, which tells you how forgiving the underlying physics is.

Analogue

Blueprints (1)

Crystal Detector