CRYSTAL JOTTINGS

Radio Record, Volume I, Issue 26, 13 January 1928, Page 13

CRYSTAL JOTTINGS

Recently the idea was mentioned of having several cat’s-whiskers bearing on the crystal, each whisker being contected to one stud of a multi-point switch. .In order to keep the cat’swhiskers in place once the sensitive spots are found, a small piece of gauze-like material is stretched over a xing, and this is fixed above the crystal, A cat’s-whisker is put througlt one hole alter another, and when a sensitive spot is found, is held there. by the gauze. Crystal detectors, especially those employing some form of galena material, are apt to become insensitive and difficult to adjust after a time owing to the surface of the crystal becoming slightly tarnished by exposure to the air. By breaking one or two small fragments off the crystal a fresh and active surface is exposed which, with most crystals, will be found to be as good as the original surface of a new crystal. The same is true of the cat’s-whisker when this is made of some base metal such as brass or a poor quality of gold which is likely to tarnish. If the tip of the cat’swhisker is cut off with a clean pair of scissors a clean cross-sectional surface is exposed; ‘this often improves the sensitivity and stability of the detector noticeably. Of course this does not apply when the cat’s-whisker is made of some. non-tarnishing metal. Tt is a well-known chemical fact

that native galena, when heated to 950 degrees in vacuo or in a current of inert gas such as nitrogen or argon, } gnelts and sublimes and collects in the form of brilliant cubical crystals. This process is often taken advantage of in the manufacture of synthetic galena crvstals, but, of course, it is considerably improved and refined in details. Another method of ‘trzating’ galena consists in rapidly cooling the mineral after it has heen melted out of contact with air. Under these conditions very fine crystals are obtained. Galena, it must be noted, is always heated out of contact with air, otherwise it would become oxidised and converted into lead sulphate-a totally different substance. A good idea which won a prize in a wireless competition some while ago: in an American magazine was to have several cat-whiskers bearing on the same crystal, the various cat-whiskers being taken to studs of a switch, so that by moving the switch arm you could choose very rapidly ofte roint or enother according to which was the est.

Many of the crystals ordinarily used as wireless detectors are sensitive to the incidence of light waves, and in farticular galena and argentite (sulphides of lead and silver respectively) slow _ this effect in a more or less marked deree. Crystals also will be affected by heat, and both heat and light have the effect of producing a difference of potential between the crystal and the metal contact. It is not known with any certainty what is the exact mechanism of these effects, but it is very probable that they are directly related to the ectifying properties of crystal contacts for alternating electric currents. The golden scale which is often fornd on certain specimens of ordinary coal is ofter quite a good rectifier of highfrequency impulses. But only the golden portion is conductive, so the mount as well as the cat’s-whisker must contact the metallic surface. The surfaces of crystals can be cleaned by placing the crystals in a bottle containing a small quantity of coarse but perfectly clean and dry sand. Cork the bottle, and then give the contents a vigorous shaking for a few minutes. ‘Any dirt or tarnish on the crystal sutfaces will be completely removed by this simple process,