Crystal Detector

A long-wire aerial near overhead power lines is lethal. String it well clear, bring it indoors through a window, and disconnect and earth it in a thunderstorm.

Set a piece of galena or silicon in a shallow brass cup, packing it with soft metal or conductive paste so the whole underside makes contact. One face must stand proud.

Do not polish or handle the exposed face. Skin oil and oxide films kill the sensitive spots, and there is no way to restore one once it is contaminated.

Coil a 30 mm length of fine springy wire — phosphor bronze about 0.1-0.2 mm — into a small helix with a straight tip. The coil provides gentle, constant contact pressure.

Fit the whisker to an arm that can be swivelled and lowered over the crystal. You must be able to reach any point on the crystal face — that adjustability is Pickard's actual claim.

The tip should just rest on the surface. Pressing hard makes the contact ohmic — it conducts both ways equally and detects nothing.

Wind about 60 turns of enamelled wire on a 40 mm tube, tapping every ten turns. This selects the station.

Run 10-20 m of wire as high and as straight as you can. A crystal set has no amplification whatsoever — the aerial supplies every scrap of energy you will hear.

Clamp to a cold water pipe or an earth rod driven into damp ground. A poor earth is the commonest reason a crystal set stays silent.

Aerial to the top of the coil, earth to the bottom. Detector and high-impedance headphones in series across a tap. Four components and no power source of any kind.

Old crystal-set headphones of 2,000 ohms or more, or a modern low-impedance pair through a matching transformer. Ordinary earbuds load the circuit dead flat.

Lower the whisker and move it slowly across the crystal, a fraction of a millimetre at a time, listening. Most of the surface does nothing; a few points are dramatically sensitive.

Note where the whisker sits. Knock the set and you lose it, and finding it again takes minutes — this fragility is exactly why the vacuum tube replaced it.

Move the connection along the taps to change the tuned frequency. Fewer turns for higher frequencies. Expect to hear only strong local stations.

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.