Battery Potential on Crystal

Radio Record, Volume II, Issue 6, 24 August 1928, Page 27

Battery Potential on Crystal

READERS often inquire as to the method of applying battery potential to the carborundum crystal. If the biassing voltage is carefully applied to the erystal and properly adjusted, results exceeding those of the ordinary perikon detector may be gained. (A perikon detector is usually zincite in combination with copper pyrites, hornite, or other crystal.) When a biassing voltage is applied there is a critical point where the strength of signals increases in greater proportion than the applied voltage. In all other crystal combinations the increase in signals is only in direct proportion to the applied voltage. Inthe case of both there are definite limits above which increase of applied. voltage will produce ne benefit. HE usual method of applying voltage to a carborundum-steel combination, such as those obtainable in eartridge form, is shown in the diagram. The ordinary tuning circuit is shown at the left, the crystal in this case being shown connected to a tap on the tuning coil, but may be connected to the aerial end if found an advantage. The circuit is tuned by a .0005 variable condenser with about 56 turns on the coil, or .00025 with about 78 turns, the wire being better spaced about half its own thickness. The diameter of coils is 3 inches. The foregoing particulars are given for those who wish to construct a complete receiver, but any existing crystal receiver may have the

earborundum cartridge put in and the biassing arrangement added. Hi biassing is accomplished by means of a potentiometer of 400 ohms resistance. ‘The arm is connected to one side of the ’phones, and the two other contacts to the negative and positive, respectively, of two small flashlight dry cells. A connection is tuken from between the two cells ta one end of the crystal and to a fixed

condenser of .0005 ‘capacity, which functions as a2 by-pass. A switch should be provided for each battery as shown, and‘this may conveniently be a double-pole single throw, so that both are switched off together when not in use. It is immaterial which end of the detector is connected to the central point of the battery, as the potentiometer allows the application of a voltage of positive or negative from 0 to 13 volts. If one cell is omitted its position is bridged over with wire, and it is then

necessary to find out: the correct position of the detector by 1eversing it. Only one side of the switch would then be required. Complete units comprising the crys-

tal, potentiometer, by-pass condenser, and dry cell can be purchased at the dealers.