234' 



ANNALS NEW YORK ACADEMY OF SCIENCES 



for good tuning the inductance of the coil L^ must be made extremely 

 large to secure the necessary ratio of the reactance of Lq to the resistance 

 of the circuit. This disadvantage is overcome in the system of Fig. 27 

 by removing the telephones from the audio frequency circuit, and using 

 the latter to operate a second audion. The telephones may then be 

 placed in the wing circuit of this audion without adding appreciably to 

 the damping of the circuit. The tuning of the circuit L^Cq may there- 

 fore be made very sharp with reasonable values of inductance simply by 

 keeping the resistance low. In this case considerable amplification is 

 obtained by the use of resonance in the transformer M^ to increase the 

 voltage impressed on the grid of the second audion. The great advan- 

 tage of this kind of tuning is shown by the following example. Suppose 



Fig. 27 



the incoming signal has a frequency of 50,000 cycles, and the local fre- 

 quency is 49,000 cycles. The differential frequency is 1,000, and the 

 audio frequency circuit is tuned accordingly. An interfering wave 1 per 

 cent shorter than the signaling wave, of 49,500 cycles, will produce an 

 audio frequency of 500 cycles per second, which will not appear at all in 

 the wing circuit of the second audion unless it is many times stronger 

 than the 1,000 cycle signal. This combination of radio and audio fre- 

 quency tuning is too selective for use at the present time, even when the 

 sending station is equipped with an alternator, as the slight changes in 

 frequency of the radiated wave produce changes in the beat frequency 

 of the receiver which carry it out of range for the sharply tuned audio 

 frequency circuit. A disadvantage of this method of tuning is that at- 



