WIRELESS TELEPHONY FESSENDEN. 



185 



Fig. 4. — -Circuit for 

 use with condenser 

 transmitter. 



Olimic resistance of receiver, 2,500 ohms. 



Value of high frequency current necessary to produce readable 



signals, 6 X 10~^ amperes. 



Electromagnetic wave energy required to produce audible note 



for period of one second, 1 X 10"^ ergs. 



The telephone used for detecting the signals 



had a resistance of approximately 1,000 ohms. 



Some measurements were made to determine the 



change of current in the telephone circuit by 



using a sensitive galvometer in series with the 



telephone, but the results obtained were obviously 



too low, possibly on account of the electrostatic 



capacity of the turns of the galvanometer with 



respect to each other. It will be noted that the 



amount of electromagnetic wave energy necessary 



to produce a signal is considerably less than that 



given in a previous note.*^ The difference is possi- 

 bly to be attributed to improvements in adjustment and operation. 

 The above measurements were taken by shunting the barretter 



across a piece of straight resistance wire in series wdth a hot-wire 



ammeter, to determine the voltage necessary, and by introducing 



resistance in series with the barretter to de- 

 termine the resistance of the barretter. The 

 figures were also checked in a number of 

 other ways and very concordant results were 

 obtained, so that it is believed they may be 

 relied upon. 



The previously mentioned thermoelectric 

 receivers or rectifiers of Doctor Austin and 

 Mr. Pickard, shown in plate IG, figure 2, and 

 the vacuum tube receivers of Fleming, De 

 Forest, and Cooper-Hewitt also act very satis- 

 factorily. The fact that the writer has not 

 been able to get as good results from them 

 may be due to greater familiarity with the 

 liquid barretter and heterodyne receiver. 



Plate 17, figures 1, 2, and 3 show forms of 

 heterodyne receiver adapted for use for 

 telephonic work. 

 Receiver connections. — ^Where the wireless telephone is operated 



by first talking into the transmitter and then throwing a switch and 



listening, the usual wireless telegraphic connections are used. This 



has been found in practice to be very inconvenient, however, and 



Pig. 5. — Circuit ; modulation 

 accomplished by changing 

 inductance of one of oscil- 

 lating circuits. 



" Electrical World and Engineer, October 31, 1905. 



