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 



147 



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- 



LVWVV VWW 



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- 



