Semiconductor history

The use of natural semiconductors

I've started collecting remnants from the era of crystal radio (1900-1930) - partly to use to explain what I'm doing in performances but mostly to learn some of the lost history of this technology. There is plenty of information out there, but a lot of the practical details are missing. This is interesting to me after doing this DIY for so long, seeing how it worked when these techniques were widespread.

Tins, cat's whiskers and crystals

It was common for crystals and cats whiskers to be sold in these tiny tins (and at this time the semiconductor industry was thriving in Britain). I think there are two types of springs here, the ones on the left were used for tensioning the screw and the ones on the right are actually the cat's whiskers that touched the crystal surface. The pressure is critical in making a good point contact junction, so they would have been specially made to provide the correct resistance. I'm not sure but I think the little plastic pill containers were not contemporary as in the 1920s when these tins were made, plastic was confined to bakelite and ebonite.

This is a tin that contained a crystal of "Hertzite". There were many brand names but usually it seems these contained galena, which was considered one of the best for radio detection - as it was more 'selective' (could be tuned to only pick up the station you wanted to listen to) than other minerals. I'm not sure where the galena would have originated from but presumably being based in Gateshead maybe nearby County Durham which has lots of lead mines such as Blackdene Mine. I'm also curious as to what they used to mount crystals.

The 'perfect' crystal - interesting though that they used silver wire for the cat's whisker. Image from the excellent radiomuseum.org

Neutron were a company that was immensely popular for a few years - apparently following 16 months of R&D they developed a crystal that "doubled the range" of a radio receiver. Inside the tin we find several galena crystals, all bright and fresh - I've often wondered how long it takes for fresh galena to tarnish, it seems these have lasted well since the 1920s. Apparently the best galena was argentiferous, or silver bearing. Some of the Cornish mines also reported silver sales from galena processing.

Radio sets and crystal diodes

Crystal radios from this time are still possible to buy, but for quite high prices. I managed to get this front panel for much cheaper than the complete sets (they come in nicely made wood boxes that are popular to reuse for other things). It has a giant variable capacitor for tuning, some screw terminals and an intact crystal/cat's whisker setup.

There were hundreds of companies that sprang up to take advantage of the radio craze of the 1920s, producing thousands of models of crystal radio. It was the cheapness of the solid state semiconductors that made radio accessible to huge amounts of people, especially as you could make them yourself pretty easily - with no batteries required!

I've been trying to work out which model it is and which company manufactured it. In terms of non-symmetrical layout and the style of the tuning knob it looks most closely like an Ericsson Telephones model (yes that Ericsson, or rather it's British subsidiary) but the faded label in the crystal holder glass envelope has a logo just about visible that looks the GvR. This could potentially be General Radio London, who were active at the time, but I haven't managed to find a logo that matches. It's also possible that the crystal holder was made by a different company, or that the radio was a knock-off copy of an Ericsson - a lot of that went on.

The raised plug at the top is where you plug in a secondary inductor coil which could be bought separately. The panel is probably made from bakelite and the knob and plug block is perhaps ebonite - both early forms of plastic that would have seemed like dazzling and futuristic materials at the time.

I was so happy to get such a good example of this kind of crystal setup - a precursor to all diodes to come! The cat's whiskers match the ones in the 'mighty atom' tin, and the pressure on the crystal is quite a lot more than I thought it would be. The handle end has a ball bearing so you can move it around to find a working spot. I think the cat's whiskers are made from silver, which apparently works best with galena. The crystal itself seems too dark for galena, but on close inspection it shows cubic formations and the hardness is a match so I presume this is tarnish. The top surface is scratched into rounded masses, but there was an area on the side that had been protected showing these original crystals.

This design is really well thought out as the whole thing can be dismantled easily, so the cup containing the crystal can be replaced or the cat's whisker changed. It works so much better than my attempts, as it holds much more securely to the location you pick, and the glass will protect it from dust.

I've managed to carefully remove the crystal holder from the radio panel (and kept all the screws and bolts for safe keeping so I can rebuild it) and put it on a wood base, as the enormous variable capacitor (which give a range between 0.7 and 1.3 pF) was making it cumbersome.

I then tested to see what curve traces I could get from it - exciting to pass some voltages over this crystal for perhaps the first time in a century. It's not that different from what I've been used to seeing, which in itself is interesting - I had wondered if I was replicating this properly for a while. The curves represent different spots on the crystal, and you see some different behaviours - and a bit of inverse curves which I've seen from galena before.

The top right is typical of it 'properly' performing as a rectifying diode, the voltage jump in the positive region on the right happens at a very low voltage, and rivals a germanium diode. It is quite leaky though, as it starts conducting (although, with some resistance) at higher negative voltages.

The advantages though with this design is its stability, something I've not really been prioritising - but it does stay quite well 'set' once you find a spot. Having said that, knocking it, or the table still causes jumps and changes in states so the radio must have still be tricky to keep working.