With the Crystal User

Radio Record, Volume I, Issue 23, 23 December 1927, Page 12

With the Crystal User

USEFUL POINTS FOR BETTER RECEPTION

If you would maintain your galena crystal in tip-top condition, use it in a glass or celluloid enclosed detector. It is not so much the dust in a room which effects its sensitivity, but the impure atinosphere to wluch it is generally exposed when it is employed in a detector of an open type. A good way of storing a spare crystal or two is to bury them in a little box containing perfectly pure white sand. See that the sand is quite dry to begin with, and then place the box in a perfectly dry place. ‘Treated in this manner any good crystal will retain its initial sensitivity for au almost indefinite period. If you are out of long-range work with a crystal set, use a fairly coarse gram galena crystal (preferably one of the "natural" as opposed to the synthetic variety), with a fine graphite of soft lead pencil contact. Tor ordinary work, however, where the crystal set is not very far away from the broadcastiny station, there are very few things to beat a zincite-tellurium contact mounted in a "perikon" detector. With initially strong signals, this device gives loud and clear reception, and it does not require very much adjusting. ‘this is the usual semi-permanent type of detector. ‘Unsatisfactory valve-crystal reception may very often be due to the use of an unsuitable crystal. ‘Try a small piece of iron pyrites, and your troubles will ccase. Iron pyrites is rather difficult to obtain in a perfectly satisfactory sensitive condition, but if you can get hold ee eee ao am hanna |

of a piece of this mineral you will find that it will give clearer reception than any other. OSCILLATING CRYSTALS. Perhaps you are interested in the subject of crystal oscillation, and you have been experimenting with steel-zincite contacts? However, there are other metals which will give rise to oscillations in the circuit when they are used in contact with zincite. Cromium aud manganese are two. Cobalt is another. There is room for innumerable expetinents in this direction. On the seashore you sometimes see a number of small pebbles which possess a greenish-grey ~netallic IJnstre. Next time you have the chance, take a number of these home with you, and try them ont in contact with zincite. In many instances you will find that excellent rectifying contacts are provided. MAKING YOUR OWN, ‘The very simplest ‘crystal’? fo make is obtained by placing a strip of copper in a red fire for about half a minute, or even less, until its surface has. acquired a greyish-black appearance. Allow the metal to cool down to ordinary temperatures, and then test its rectifying properties by means of a very lightly applied No. 386’s wire cat’s- whisker. You will be able to get quite good reception. LTurther, the strength of the reception is still more increased by having, as the contact point, a piece of stout copper rod, the end of which has been dipped in molten rock sulphur for a minute or-two, and then heated in a non-luminous gas flame so as to drive off the excess of sulphur. USE A FIXED CONDENSER. Always employ a small fixed condenser across the ’phone terminals. With some crystals the degree of improvement which it brings about in the r ception is striking. Choose the capa ity of the condenser well, however. I the latter is too large there will be a damping effect, and the signal strength will decrease instead of increasing.