Mohave County miner and our mineral wealth., 1922-06-23
RADIO
J. F. Davidson


 We will first consider the reception of damped waves, It will be remem- bered that in the previous articles it was stated that radio waves are ether waves having electrostatic and electro magnetic lines of force. As the cleo tromagnetic lines of force sweep the antenna of the receiving station, os- cillationsr are started by electromag- netic induction. As the electrostatis lines of force sweep the antenna, an electromotive force is induced in the -antenna circuit. As the impressed -electrimotive force varies and changes direction, due to the alternations in the transmitting circuit, the induced potential varies in amount and also changes direction. The induced poten- tial therefore has the same frequency as the radio wave which is being re- ,ceived. The combined effect of the electromagnetic and electrostatic lines of force causes, therefore, oscillations in the receiving antenna circuit which have the same characteristics as those being radiated from the transmitting station. .. The apparatus required to receive and detect radio waves is similar, to some extent, to that usedin the trans hitting station. The same type of .antenna may be used. An oscillation transformer similar to the one used in transmitter is employed to trans fer the received oscillations from the antenna circuit to the local oscillatory circuit. This transformer is somewhat different in construction than that used in the transmitted. The receiv- ing oscillation transformer is compOs- ed of one or two coils of wire, de- pending as to whether or not con ducive or inductive coupling is used. The wire, about size 20, is wound closely on a small cardboard or fiber tube, while the transformer used in a transmitter is made of strip copper or heavy copper wire. NThe action, however, is the same in both cases, the oscillations in one circuit induc- ing oscillations in the other circuit The middle section of receiving cir cult is nearly the same as the trans hitting circuit, and is composed of an induction coil (one part of the oscil' lation transformer) and a condenser No spark gap is used in the receiving circuit. A telephone receiver and "a crystal detector are connected in series with the induction coil and the con denser. We now have complete damped wave receiver. By means of a condenser;placed In the antenha Cir cult, we are able to adjust that circuit so that it will respond to any given frequency of waves which may sweeI the. antenna.
 Suppose the transmitting station i: radiating waves which have a lengtf of 200 meters; that is, the oscillations which are being radiated have fire quency of (30O, O,000,000 divided by 200. 100000. In order to receive thes, waves we first adjust the antenn: circuit of our receiving station so tha it will have this same period of oscil lation. This adjustment is made b! varying either the capacity. or thu amount of inductance in the primar. side of the oscillation transformer, O. both. We then tune, in similar man ner, the secondary circuit of our re ceiver, so that the two circuits ar in resonance. Now when the radiate, waves from the transmitting station sweep the antenna of our receivin, station, oscillations will be set up il the receiving antenna circuit. Becaus the primary and secondary circuits il our receiver is in resonance, these os cillations will induce similar oscilla tions in the secondary circuit, and tn received signals will be heard in tn telephone receiver. The human ca responds only to vibrations which hay frequencies between 30 and 5000 These frequencies are therefore calle audo frequencies. The signals receiv ed on a wave length of 200 meter have frequency of 1,500,000. A such high frequency it is therefor evident that the ear would be unabl to give us the impression of sount In order to make the signal audible the frequency must be lowered. Thi is the function of the detector.
 It has been found that certain min erals have the property of permittin current to flow through them only i one direction. Galena crystal is on of these minerals. It will be remen bered that wave train is made up 0 a series of oscillations, and that ther is a reversal of the current durin each of the individual oscillations. crystal of galena is placed in serie with the telephone receiver in the sex ondary circuit, it is seen that durin each wave train current is caused flow through the telephones only dul ing one half of each oscillation, tn peculiar property of the galena pel mitting current to flow only in one d