RADIO AND ITS RECEIVERS

Sun (Auckland), Volume I, Issue 209, 23 November 1927, Page 14

RADIO AND ITS RECEIVERS

CRYSTAL CIRCUIT THAT OSCILLATES

SOMETHING NEW FOR EXPERIMENTERS

The fact that under suitable conditions a crystal detector could be made to oscillate, was discovered by Dr W. H. Eccles, FJt.S., several years before the war. Little is known of the action of the crystal, the valve having been the centre of focus, but some scientists, however, are returning to the earlier device with the object of ascertaining the reasons for the rectification and oscillations of the crystal. To the experimenter the oscillating crystal otlers a iuitful and comparatively inexpensive field for experimentation, and the circuit described here by Mr C. R. Russell, M.Sc„ is one which has been found to produce oscillations.

The circuit described is given for those readers who prefer to experiment rather than to listen. When suitably handled the circuit is capable- of oscillating and will pick up the carrier wave of a fairly distant station; before this condition is reached there will be a marked increase in amplification. PARTS REQUIRED. Two variometers, one carborundum or Perikon detector, the latter being preferred, one .00025MF. variable condenser, one .001 fixed condenser, one .0005MF ditto. The variable condenser shown in aerial circuit can be replaced with a fixed one without affecting results. One 400 ohm potentiometer, and two 4-volt flash lamp batteries are required. The two variometers can be mounted on a piece of wood about six inches

apart, or can be mounted on a panel as desired. The aerial terminal (which should with the earth terminal be mounted on an ebonite strip) goes to the .0005MF fixed condenser thence to variometer, one side of .00025 variable condenser and to crystal. The earth terminal goes to variometer, thence to rotor plates of variable condenser and to the centre of the batteries, which are connected across potentiometer. A switch should be provided to disconnect batteries when set is not in use. The centre arm

of potentiometer goes to one phone terminal, while the second terminal goes to the second variometer, whose remaining terminal goes to crystal detector. A .001MF fixed condenser-is connected between centre of batteries and phone terminal connected to second variometer. OPERATION. Tune in the local station by operating the left variometer, potentiometer arm in centre, variable condenser out, then try moving the second variometer. Get the loudest signals, then move potentiometer and note effect. When the local has closed down, try searching for a more distant station and after one has been heard weakly, adjust potentiometer, move second variometer and very carefully adjust variable condenser and note if a hissing sound appears or whistles occur when condenser is

moved: if not adjust potentiometer until the hiss occurs; set is now oscillating. Below the oscillation point there should be a considerable increase in signal strength, and with oscillations taking place distortion of the music. It should be understood that it takes careful manipulation to produce oscillations, the detector adjustment being very important. Double crystal detectors, such as Zincite and Pyrites probably give the best results, although many other crystals will oscillate if the correct bias is given.