Our Crystal Corner

Radio Record, Volume II, Issue 38, 5 April 1929, Page 31

Our Crystal Corner

By

Galena

yr your reception is interrupted intermittently according to the position of your head or the position of the telephones, it is a sign that one of the leads is faulty and needs replacing. ' An excellent test for sensitivity is to. Jace the telephones over the ears in he ordinary way, put one of the tags between the lips and rub the other tag with a key, nail, or other piece of metal; if a rubbing noise is heard corresponding with the movement of the key you can be sure that the ’phones in question are sensitive. As telephones are among the most sensitive instruments known to science that are in commonm use, we should always treat them carefully. When telephones are not left connected permanently to the set, but are stowed away in a cupboard, make sure that they are kept in a dry place, as dampness will in time affect them. If the telephones are incorrectly connected to the phone terminals in a valve set, it is only a question of time before they become demagnetised, owing to the plate current flowing through em in the wrong direction. It is easy to damage telephones by dropping them for-apart from the effect upon the magnetism-their sensitivity depends upon the exact shape and position of the diaphragm. a: telephone diaphragm is discovered to be rusty, it can after careful removal be cleaned and slightly smeared with petroleum jelly before being replaced, in order to keep it in good condition. If, for any reason, the telephone diaphragm is taken from off the earpiece be very careful not to bend or damage it in any way. When replacing a diaphragm over a telephone earpiece, do not place the diaphragm straight down over the earpiece, so that the magnet exerts a pull upon it, but slide the diaphragm sideways over the rim of the earpiece so that it will not be bent or pulled. allowance for use with the telephones. as very much enjoyment may be had by other members of the family when they can pick up the phones and listen when and where they @. How the ’Phones Work. ‘THOSD who have unscrewed the ear eap of the telephones have seen a coll of fine wire wound around an iron stud. The latter is the pole of a permanent magnet. A permanent magnet will always exert a pull upon certain objects such as the metal of which the diaphragm of the telephones is con‘structed. 4. coil of wire woynd rouvf" such a __,magnet and carrying a current of elec- \ tricity will influence the intensity of ‘the pull of this magnet. The wireless waves are really variations in voltage impulses, and these variations make themselves felt in the wire surrounding the magnet, which in turn passes the voltage impulses on to the diaphragm, as it is either attracted or repelled according t& the intensity of

the signal passed. This fluctuation ef current then causes a similar fluctuation of the telephone diaphragm which causes a variation in the air, and this variation is caught up by the ear and so the sounds are heard. Galena Orystals. LENA, of which many of the detectors in erystal sets are made, is a natural sulphide of lead, crystallising in both cubic and octahedral crystals. In the massive form it occurs widely distributed throughout the world as primary metallic veins in limestone, or as irregular deposits, Almost all natural galena is radio-sensi-tive to a certain degree. Hard specimens of this mineral are generally more sensitive than softer ones, and those with a slightly striated surface will be found remarkably so. The most sensitive spots are to be found at the very angles of cubic crystals or clearages or on minute octahedral clearage planes. The best. results are obtained by connecting the crystal itself to the "ground" side of the circuit and the cat’s whisker to the aerial side. The Full-Wave Variometer Set. *X Y.Z." (Mahanga Bay) who has * constructed the full-wave crystal set, writes:-"My results are de- cidedly poor, and by no means as good as what I am getting on my present set with one permanent detector. I have tested both carborundums and cannot detect the least difference in either. The aerial directional to 2YA is 200ft., and the earth two copper plates. In tuning, the rotor can be placed anywhere with little alteration in volume. What amount of wire should be on the stators with my long aerial?" A: It appears that more wire could be added to the rotor or the stator, for there is no doubt that the fields created by both the coils are not strong enough to sufficiently influence one another. Increase the coupling between the aerial and the stator, Details of a Tapped Coil, "S H." (Dunedin) asks for details. of * aI tapped basket coil consisting of 60 turns. He has already one of 38 turns, and ean tune in the local stations with very good results. A.: As the inductance of the basket type of coil is very high, there is no advantage in exceeding 60 turns. Proceed in the same way as described in the "Radio Record" of December 14, 1928, and tap every: 10 turns, that is, clear away the insulation, and solder a piece of heavy wire, say 18 guage, at this point. Shellac the joint or wrap insulation tape round it. Wach of the tappings can be in turn connected to the aerial, while the condenser is connected in parellel across the whole length of coil. By this means, a great tuning range can be covered, The correspondent has asked for a diagram, but if he constructs the basket coll according to instructions, he

should have no difficulty. If trouble is experienced in connecting the different tappings of their respective destinations, he should refer to the description of the "R.R. Selective Crystal Set" described by "Megohm" some time ago. By substituting the basket coil for the plain solenoid coil, every-~ thing should be made clear, The Speaker for a Crystal Set. WHEN a one or a two-valve amplifier has been added to a erystal set, eare should be exercised in selecting the speaker that will give the best results. It is indeed rare that a very great output has to be handled, so that capacity to handle volume is unnecessary: in fact, it is a distinct disadvantage. A small sensitive speaker, preferably of the cone type, gives greatest satisfaction. Success of Double Grid Valve and Full wave Crystal Set. I HAVE made up a crystal set using pentodes system of dual tuning and "Galenas" double-grid valve for the amplifier described in article (crystal set and amplifier to work off 22% volts. The volume was wonderful for a crystal set. When made it developed a whistle, and was very rough, but that was cured by shielding coils and putting .001 condenser across primary of transformer; .the performance was equal to a two-valve set . I received IYA, 2YA, 8YA, 2FO, 2BL, LZQ, 1ZB,

AR, 3LO0, on a Lott. antena.-

C.

F.

Mickle

(Auckland).