Text: Theory of operation of a device used to detect radio waves, usually comprising a crystal in combination with a metallic contact, or, in some cases, two crystals arranged to rest against each other. In action this device serves to rectify the incoming signals, that is to say, it changes their nature from an alternating current to pulsating direct current. The incoming waves or signals consist of a series of alternations, the current flowing first in one direction, then reversing and flowing in the opposite direction along the circuit of the receiver. The changes of direction take place many times each second and would not be audible in the head phones in their original state. The crystal, as used in radio reception, has the peculiar property of furnishing a ready path for the flow of current in one direction while offering high resistance to its passage in the other. Now an alternation (q.v.) is a rise in current from minimum to maximum in one direction and back to minimum or zero, repeating the process in the other direction. Two of these alternations, one in a positive direction and the other in the negative direction constitute a cycle. (q.v.) If the crystal will permit the passage of current in one direction only, it is apparent that only half of each cycle can pass through; the other half of each cycle being lost or blocked out by the action of the crystal. The illustration "A" gives a graphic representation of the radio frequency oscillations as they move over the aerial circuit. The curves above the line show the current in a positive direction while those below the line show it in the opposite or negative direction. If the signals or waves in this state were sent through a telephone receiver or head phone in a radio receiving set no sounds would be reproduced. Then again if the incoming oscillations were continuous, of unvarying strength, the rectified current would merely act on the diaphragm of the phone and hold it in a certain position as long as the signal persisted. This of course would not create any audible or intelligible sounds. If, however, the incoming oscillations are in the forms of groups or trains of waves, in any one of which the successive oscillations were less than the previous ones, the current would be rectified as shown in B and the result would be a series of pulses in the phone as indicated in C. These pulses would occur as often as the trains of oscillations occur (for more complete explanation of the difference between continuous and damped waves, see Damped Waves, also Continuous Waves ) . It will therefore appear that a crystal detector can only be used to receive damped waves. When the rectified portion of a damped wave train reaches the phones, the diaphragm is depressed and held until the train has died out, at which point the diaphragm is released until another train reaches it. This creates the audible signals and the number of trains ococurring each second will determine the number of depressions of the diaphragm and hence the pitch of the audible signals. ( See Crystal Receiver, Crystal Detectors, also Tickler.)

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Source: 190