ADVICE ON CRYSTAL

Radio Record, Volume I, Issue 28, 27 January 1928, Page 5

ADVICE ON CRYSTAL

GALENA POPULAR. If you go into a wireless shop and ask bravely for a crystal for your re« ceiver, the assistant will immediately ask you what kind you require, says a writer in ‘‘Modern Wireless." If you say, "What kind do you keep?" he will reel off a list of names as long as your arm. Some of these names you will recognise as those of mineral sub« stances with which you are already acquainted, such as silicon, gaiena, cop« per pyrites, iron pyrites, etc. You will. find, however, that many of them have such names as hertzite, lionite, permanite, electronite, markonite, and What-not. If you are not experienced in using crystals you can do worse than choose one of the crystals with a fancy name, for they are practically all specially treated galena, itself..a very good rectifying substance. The special crystals are greatly superior to ordinary galena in one very important point-the number of sensitive spots te be found on their surface. RS Sensitive Spots. If you take an ordinary cubical crystal of galena you will find that it ts very bright and shiny and smooth, On the smooth surface you will be able to find two or three exceedingly sensitive places, but they will take a good deal of finding. ‘The specially treated crystals, on the contrary, are sensitive on practically every part of their surface. If you examine such a crystal under the imcroscope, using a low power, you will see that the surface, consists of a very large number of tiny crystals, and as each of these tiny crystals has one or two sensitive spots. on its surface, you can easily see why the whole substance must have a_ very large number of sensitive places. The Whisker. There is a great deal in the proper choice of the cat-whisker. By the way, I wonder who was responsible for the peculiar term?, I don’t see why the fine wire we use for making contact with the surface of a crystal should be called a cat-whisker any more than a pig’s bristle or a dog’s hair. However, there it is. The name is stuck fast in wireless literature and cannot be removed even by force. This cat-whisker must be of metal, and it is advisable that the wire of which it is made should be as fine as possible. The end of the wire which touches the crystal surface must be kept quite bright, as . such nietals as copper and brass oxidise or have some otlier film deposited upon their surface from alinospheric action. If we are to retain our crystal detector at its maximum sensitiveness, we must occasionally keep this surface bright by clipping off the ends of the wire, Try Gold Wire. If you take my advice you will buy a gold wire cat-whisker. These are very cheap, as the gold is only 9-carat, and being fine very little of the material is used. This cat-whisker costs about 4d. to 6d., and has the great advantage of keeping bright at its point for very long periods, if not indefinitely. A thick cat-whisker will often miss the most sensitive points on the surface of the crystal, while a thin one will ferret out many which otherwise you would not notice. Thick whiskers can be improved by sharpening the point with a file.