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The Cat's Whisker.
By Anthony

The detector used in a traditional Crystal Set is simply a point-contact semiconductor rectifier, albeit a relatively primitive and unstable one. The crystal normally employed is galena, the common sulphide ore of lead, used without treatment directly as it is mined. Two Galena Crystals Crystal

It is quite easy to make up a modern version and much fun - and aggravation - can be had from constructing one. More advanced proprietary versions of 'permanent' detector were sometimes used; they consisted of various combinations of pairs of different crystals in fairly heavily spring-loaded contact. All the specimens I have encountered have worked fairly well and were certainly much more stable than the conventional whisker type, but less sensitive and also prone to contamination and damage from dampness. [2]

A small piece of crystal about the size of a large pea is used, open to the air. [3]

Effective mounting with proper contact to this crystal is not easy to achieve. Originally one used a small brass cup, with some form of terminal for the connection; this was part filled with a very low fusing-point alloy such as Wood's metal or Mollets's metal, heated just to melting and the crystal dropped in. These alloys are not commonplace.[4]

Some success may be had by crimping a thin brass cup carefully as tightly as possible all round the middle of the crystal, though this can be a source of instability of contact which is to be avoided; and I have also tried using that silver-loaded conductive paint sold in tiny tubes from electronic component suppliers for such purposes as repairing broken tracks on car rear-window de-misters and the like. I used this method only twice - once it was a great success, and once a dismal failure. Whatever method is tried, take great care to have all contact surfaces scrupulously clean.

The contact Cat's Whisker is merely a springy piece of thin wire, mounted on a suitable insulated holder, and used to probe the surface of the crystal for a 'sensitive spot'. This can be very frustrating, as a good contact can so easily be lost by the slightest vibration. One needs hours of patience! The brass wire taken from a soft wire brush works well as a starting point but as ever with crystal sets there is room for considerable experiment. I found that the brass 'suede-brush' wire wasn't long enough to handle conveniently, so I suggest extending it with a small very light spring. Unwind the ends and straighten them so as to leave two or three turns in the middle; fix one end to the mounting and circuit connection and then experiment with contact points soldered onto the business end of the arrangement. The whole can be perhaps an inch long but not more or it will probably wobble too much. The working end of the cat's whisker should be fairly sharply pointed but cutting the very end off with small side-cutters usually leaves a suitable working end. Different metals for the contact wire can give different results; the best results I ever had were from the point of a steel gramophone needle, but I was never afterwards able to duplicate the arrangement. Some springiness is necessary, so as to maintain enough pressure of the point against the crystal; but too much, or too long a spring such as tends to a sideways wobble, makes for dreadful instability of the contact.

The cat's whisker has to be mounted in a suitable holder so that the entire exposed surface of the crystal can be probed from many directions to try and find the most sensitive working junction. Originally special holders were produced, but a suitable substitute can be made up from part of one of those little workshop devices that are sold in cheap catalogues "for the hobbyist". They consist of a fairly heavy base (or else a suction cup fixing) carrying a variety of crocodile clips on universal mountings, and usually a plastic magnifying lens as well. One of these universal joints can be taken off and used to hold the cat's whisker; but pay careful attention to cleanliness and lubrication so that it moves very nicely and smoothly. This universal mounting should have a small insulated handle for adjustment and some sort of terminal or soldered contact point for the connection to the circuit. To revert to our theme song, "there is much room for experiment". The crystal and cat's whisker holder are then mounted close to each other on an insulating fixture of some sort - thin sheet 'perspex' or similar thermoplastic material makes a good job, if one folds a small 'U'-shaped bracket, drilled to take crystal and whisker holder, and made with small feet for mounting.

An excellent circuit for initial experiment can be found at Selective Crystal Set by P.A.Dewhurst

Use sensitive high impedance headphones, of course, or possibly a crystal earpiece shunted by a resistor. Make up the circuit as shown, using your lovely new crystal and cat's whisker, but at first cheat a bit - leave the whisker off the crystal and put a commercial germanium diode across it instead until you have the circuit working, then try finding a sensitive spot on the crystal once you have some sort of reception and know how the tuning works. [5]

Dewhurst's suggestion of using a microammeter is an exceedingly useful one, but if you happen to have a centre-zero movement, all the better - occasionally these detectors work the opposite way round for a while at certain spots, depending on contaminants in the galena. (A digital meter is not suitable here.)

There are many other crystal circuits and constructional articles of course. These days strong clear signals can be less easy to find, and Radio Four on long wave might be easier. I have a set working here continuously in rural Gloucestershire (unless one of the cats jumps about or sneezes and puts the sensitive contact off yet again) on 198 metres. Using a 20 foot outdoor long-wire aerial at only about 10 feet above ground, and an original pair of Lissen coils, it will drive an old BTH moving-iron high-impedance loudspeaker at modest but usable and worthwhile listening volume when it is quiet, aforementioned cats permitting.

A good crystal in a good circuit, with decent aerial and earth, could give surprising volume into the fairly sensitive headphones of the day. In fact, one could buy a small connecting extension panel with terminals for up to four extra sets of headphones (usually in parallel) for the whole family. Also a common trick to obtain the extra volume for communal listening was to place the headphones in a large bowl or basin on the table, or else to divide a headset to give two listeners one earpiece each.

When putting up aerials, keep in mind the question of capacity from aerial wire to earth, which should preferably be kept low. A good earth makes all the difference. Central-heating pipes are worth trying, but the mains earth connection is less successful as a rule. I got a couple of three-foot lengths of scrap three-quarter inch domestic copper pipe and banged them in a yard apart, first opening a hole with a long iron spike and sledgehammer. Use a thick insulated wire soldered on and connect them in parallel, and keep them well watered in a year such as we have just had. Or follow any of the many original published designs for a good wireless earth. It can sometimes take a very great deal of patience to get these detectors working, and they are so delicate and sensitive to vibration that the contact is very frequently lost; but cheating and getting the set more-or-less tuned in first, using a commercial germanium diode, can be very helpful. The tuning may alter somewhat on reverting to the 'proper' cat's whisker but at least one has some sort of starting point. With patience and some luck, good results are to be had from a genuine crystal set; a really good sensitive contact can quite often give superior results to an OA70 or OA81. (Silicon diodes are not suitable.)

Good luck!

Anthony



[1] The crystal is prone to contamination by grease or sweat marks from the fingers, so handle it with some form of forceps or in a wisp of new kitchen paper or the like. In fact, when bought loose, it was usually supplied in a little kit in a tiny tin box such as was commonly used for gramophone needles; the contents almost always included a tiny pair of primitive tweezers for the purpose.
[2] A galena crystal usually won't work very well in the presence of dampness or condensation, but takes no harm and will perform properly again as soon as it dries out. They don't like too much dust though. Original detectors were usually contained in some sort of glass tube, but I always leave mine open to the air and (carefully!) cover them with some sort of small box laid over when not in use, just to keep the dust off.
[3] I collected some my self a few years ago and this particular batch is relatively high in silver and seems to work especially well as a detector. It is from the recesses of the old lead-mine at Strontian, Argyll, some years ago; another original product of the mines is strontium carbonate and other mixed salts of strontium - the element was first isolated from Strontian samples and thus got its name. More recently the mines were used for the bulk extraction of barytes, which is or was used as a lubricant in deep drilling such as for gas in the North Sea. Specimens of galena from any other sources are always worth trying as well, as the activity of different specimens can vary considerably depending on the particular amounts of other trace elements present.
[4] Ivan in Prague, Czechia suggests, "Papin pressure pots use a small disk from low-melting-point alloy (about 120 Centigrade) as an overheating fuse. It should be possible to buy several such disks in a hardware / kitchen shop." I have not tried this, and 120 degrees C seems damagingly high for the crystal. But there is plenty of room for experiment. It may work well enough.
Roger Blackwell has also sent the following: Low melt alloy for mounting galena crystals is easily available from good model shops. Carr's (or other brand) solder for whitemetal (used for joining metal kit parts) is widely used by railway and transport modellers. It melts in boiling water
[5] It drove many original constructors mad - trying simultaneously to find a very delicate sweet spot on the crystal, and line up an uncalibrated tuned circuit!
[6] You may well need suitable refreshment from time to time. I recommend - what else? - a delightful ale, "The Cat's Whiskers", brewed just up the road from here. Whittington Brewery Really first class. Google seems to suggest a number of stockists.

A note from Maurice


Anthony was kind enough to send me some crystals to try out last year.
I tried quite a few different circuits and methods of holding the crystal (including the ones Anthony mentions).
As Anthony says you need patience but the results were well worth it.

Home Made Cats WhiskerHome Made Cats Whisker 2
The assembly shown above was originally an old trimmer capacitor from a HAM transmitter. After cleaning in a dishwasher a piece of Galena was fixed in position using Woods Metal (From Scientific Toys in the States).
For the whisker I experimented with a piece of copper wire soldered to the piston (Left hand of each photograph). The wire was kinked and the piston screwed in until the end of the wire made contact with the face of the Galena.
Using a coil and variable capacitor as the tuned circuit, the piston was then screwed in (and the wire position changed as required) until a station was heard loud and clear (This took about 2 or 3 minutes).
I am now going to make up a permanent circuit and mount the components on a piece of board.This circuit and instruction will appear here as soon as possible.
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Last updated 15.5.2007



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