Page:Popular Science Monthly Volume 88.djvu/175

 Popular Science Monthly

closely analogous to that of an auto- coherer, in which incoming-wave energy' changes the resistance of a conductor and thus alters the amount of current flowing through it from a local battery. In his early experiments with the au- dion, Lee deForest is said to have used radio-active compounds in place of the heated filament, but without success be-

���Fig. 2. A heterodyne receiver, operating on the electrical beats principle

cause of the difficulty in securing suffi- cient conductivity by ionazation. The detector of the Ainsworth patent should prove a useful instrument when devel- oped to a practical operating point, since, as the patentee points out, constancy in operation may be expected and the nui- sance and expense of filaments and heat- ing batteries are done away with.

U. S. Patents 1,141,386 and 1,141,453, issued in 191 5 to R. A. Fessenden, show not only a simple "heterodyne" receiver operating on the electrical-beats principle now so widely used, in Fig. 2, but also a method for simultaneously sending and receiving with continuous waves, as in Fig. 3. Taking up the first of these, it is seen that the antenna / is connected to ground 2 through the primary of the inductive coupler j. The secondary 4 has in series with it a variable tuning inductance 66, a condenser 5 and one winding of an electro-dynamometer-tele- phone, 6. The second telephone winding 14 is coupled to a small radio-frequency alternator ly through a transformer 8, p. The dynamometer 6, 14, consists of two coils placed end to end, one of which is stationary and may have a fine iron- wire core and the other of which is mounted upon a diaphragm 7. In re- ceiving radio signals the antenna and secondary systems are tuned exactly or approximately to the frequency of the incoming waves, so that currents of this

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frequency will be induced in coil 6. The alternator, //, is then run at a radio frequency slightly different from that being received, and its current output led to coil ij. The magnetic fields of these two coils interact one upon the other ; when the currents are relatively in one direction, the fields add and the diaphragm, 7, is attracted, and when the currents are relatively reversed, the fields oppose each other and the diaphragm is repelled. This alternate adding and op- posing of fields goes on constantly be- cause of the slight difference in frequency of the two currents, and the diaphragm is moved back and forth at a rate deter- mined by the difference in the frequen- cies. If the incoming wave is of 6.000 meters length, which corresponds to 50,000 cycles per second frequency, and the local generator produces currents of 50,500 cycles frequency, the number of impulses impressed upon the diaphragm will be 500 per second. This last is called the "beat frequency" of the heter- odyne receiver, and is the frequency of the signal tone heard by listening to the telephone diaphragm, 7. No beats or impulses on the diaphragm are produced unless both currents are flowing; there-

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����Fig. 3. Sending and receiving simul- taneously with continuous waves

fore, although power from alternator // is constantly flowing, signals are heard only when waves are received on the antenna from the distant sending station. This dynamometer heterodyne gives a much louder signal than could be ob-

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