The detector
Galena is lead sulphide (PbS), a shiny grey mineral. The contact between a metal point and certain crystals lets current through mostly in one direction: that's the rectifying effect Ferdinand Braun discovered in 1874. It recovers the sound carried by an amplitude-modulated (AM) wave. The cat's whisker is a thin, springy wire, bronze or steel, on a jointed arm: you move it to a sensitive point, because not every spot on the crystal rectifies. A knock or a vibration can lose that point.
The tuned circuit
A coil (wire wound on a cardboard tube) and a variable capacitor (or a slider moving along the coil) choose the station: the circuit resonates on a single frequency, f = 1 / (2π √(LC)). Example: 240 µH and 105.5 pF give 1000 kHz. More capacitance or more turns: a lower frequency.
No battery
All the sound's energy comes from the wave picked up by the antenna: you need a big antenna (tens of metres of wire, strung up high) and a good ground. The sound is very faint: high-impedance headphones (a few thousand ohms) or a piezoelectric earpiece are what's given, not an ordinary loudspeaker (according to Wikipedia, to be checked against a manual of the time).
The pioneers
- 1874: Ferdinand Braun discovers asymmetric conduction between a crystal and a point. He shares the 1909 Nobel Prize in Physics with Marconi.
- Jagadish Chandra Bose filed in 1901 a patent for a point-contact detector (US 755,840, granted in 1904) and used galena on millimetre waves: a pioneer, not the sole inventor of the crystal set.
- 1906: G. W. Pickard obtains the patent for a point detector on silicon (patent no. 836,531, filed on 30 August, granted on 20 November), then sells crystal detectors. At the end of 1906, Dunwoody patents a carborundum detector.
The 1920s
On 6 November 1922, Radiola, the SFR's station (Émile Girardeau), the first French private station, starts regular broadcasts; it would become Radio-Paris (in 1924 according to most sources, in 1929 according to another). The Eiffel Tower broadcasts programmes too, Radio Tour Eiffel: its very first dates are still to be checked. There are dozens of French crystal set models, such as Ducretet's (1923–1925) or Nelson et Cie's (1923).
From the crystal to the diode
In the 1940s the point-contact germanium diode (1N34) uses exactly the cat's whisker principle, sealed in glass. Diode, transistor, chip: the whole semiconductor family.
And today?
French long wave has fallen silent one by one, notably: France Inter leaves long wave on 1 January 2017 (the Allouis transmitter, 162 kHz, stays for the time signals), Europe 1 (183 kHz) at the end of 2019, RMC in March 2020, RTL (234 kHz) at the end of 2022 or early 2023. The last French medium-wave transmitter, Bretagne 5 (1593 kHz, Saint-Gouéno), went off on 31 December 2025: since then, no French AM radio left. Radio 4 leaves long wave (198 kHz) on 27 June 2026. In France a crystal set picks up almost nothing: it's an object of history. To listen to those bands, try a WebSDR, a receiver online you tune from your browser.
Listen, never transmit A crystal set only listens. Don't try to build a transmitter: transmitting without authorisation is forbidden; only licensed radio amateurs transmit, on their bands, with compliant equipment.
Safety If you put together a crystal receiver with an adult: never an antenna near a power line, never a connection to the mains, nor a ground taken from a socket or a gas pipe; in a thunderstorm, antenna disconnected and grounded, outside.
In the game The stations to tune, the crystal's map of sensitive points and the 1925 dial are made up for the game. The formulas are the real ones; the dates and frequencies come from the sources, with their reservations where there are any.