338 ANNALS NEW YORK ACADEMY OF SCIENCES 



audible all save very strong signals. By arranging to detect the oscil- 

 lations in a second audion system coupled to the wing circuit of the first, 

 interference of this sort is avoided, as the circuit L^C^ has a very high 

 impedance for the audio frequency currents and the effect produced 

 through the magnetic coupling of L^ and L^ on the second system is neg- 

 ligible. The capacity current between these two coils through the tele- 

 phones to ground is, however, appreciable; and to avoid this it is ad- 

 visable to ground their two adjacent ends as shown. The action of the 

 system may be summed up as follows. The first audion system varies 

 the amplitude of the incoming radio frequency oscillations at an audio 

 frequency, and the second audion system amplifies and detects the radio 

 frequency oscillations supplied to it by the first system. Diagram- 

 matically, the phenomena occurring are as illustrated in Fig. 30. The 

 'system gives about the same response as can be obtained with a single 

 audion working with the beat method of reception. The advantages 

 derived from the heterodyne method of amplification and the dependence 

 of the audio frequency note in the receivers on the wave length are, of 

 course, lacking ; but for the reception of waves having a frequency higher 

 than that at which beat reception is practicable, this method is of value. 



Effects op Atmospheeic Distuebances 



A very interesting feature of these receiving systems is their behavior 

 under conditions of severe atmospheric disturbances, particularly when 

 used for receiving continuous waves. Their success under such condi- 

 tions is due to the fact that they combine, in addition to their inherent 

 property of responding more readily to a sustained wave than a strongly 

 damped one, the characteristics of the two most effective static elimina- 

 tors known, the balanced valve and the heterodyne receiver. The func- 

 tion of the balanced valve is a physiological one, as it simply provides 

 a means to shield the ear from the loud crashes which temporarily im- 

 pair its sensitiveness for the relatively weak signals. In effect, it puts 

 a limit on the noise which can be produced in the telephone by a stray, 

 regardless of its amplitude. Now the effect of the static on an audion 

 is to build up a negative charge on the grid, reducing the wing current, 

 and the limit of the response which can be produced in the telephones is 

 reached when the wing current is reduced to zero. Under ordinary con- 

 ditions, this limit is too great to do much good; but when the audion 

 is generating, it is possible by proper adjustment of the amplitude of 

 the local oscillations to reduce the wing current to a point just above the 

 lower bend in the operating characteristic so that the audion is rendered 



