WIBELESS TELEPHONY — FESSENDEN. 177 



Duplex and multiplex methods. — A considerable number of these 

 have been worked out, mostly operating either by balance methods " 

 or commutators.^ It is impossible to discuss all the various improve- 

 ments, such, for example, as the method of indicating the busj^ and 

 free state of a station, the methods of sending and receiving in one 

 direction, the various types of aerials used for receiving the other 

 components of the electromagnetic waves besides the electrostatic 

 component, etc, 



Plate 3, figure 1, shows the harmonic interrupter for determining 

 the variation of intensity with change of note. 



Plate 3, figure "2, shows a type of receiver described in United 

 States patent No. 706747, in which the telephone diaphragm is formed 

 of thin copper and repelled by a fixed coil having a resistance of 

 about IG ohms. The principle of this receiver was discovered by 

 Prof. Elihu Thomson. It has been used for wireless telephony for 

 a distance of 11 miles with fairly satisfactory results. 



Plate 3, figure 3, shows a transformer used in the transmitting cir- 

 cuit. Tlie number of primary and secondary turns can be altered 

 continuously, and also the degree of coupling. The wire is wound 

 off from an insulating cylinder onto a cylinder of copper, and the 

 cylinder of copper, forming a closed circuit secondary of the trans- 

 former, annuls the inductance of that portion of the wire wound upon 

 the copper cylinder. 



Plate 4 shows a group-tuned call ; that is, a vibration galvanometer 

 which operates a selenium cell and rings a bell when a call is received. 



Plate 5, figure 1, shows an apparatus for determining the best shape 

 of coil for use with the heterodyne receiver. 



THEORY OF WIRELESS TELEPHONY. 



For wireless telephony three things are necessary : 



1. Means for radiating a stream of electrical waves sufficiently 

 continuous to transmit the upper harmonics on which the quality of 

 the talking depends. 



2. Means for modulating this stream of waves in accordance with 

 the sound waves. 



3. A continuously responsive receiver, giving indications propor- 

 tional to the energy received and capable of responding with sufficient 

 rapidity to the speech harmonics. 



Work on the wireless telephone was commenced before a satisfac- 

 tory means was discovered for producing sustained oscillations. 



To ascertain the number of sparks per second which was necessary 

 to determine articulate speech, a phonograph cylinder was taken 



« United States application No. 366528, April 5, 1907. 

 ^ United States patent No. 703652, April 6, 1905. 



