148 THE ELECTRIC ARC 



of the microphone causes a change in the current through 

 the arc. 



In making a receiver for this kind of wireless telephone 

 use was made of two facts; first, that with each change of 

 the current through the arc there is a change in the intensity 

 of the light of the arc; and second, that the electrical re- 

 sistance of selenium changes whenever the intensity of the 

 light falling on it changes, so that selenium may be used 

 to detect changes in the current through the arc. This 

 is done by making the selenium one of the arms of a Wheat- 

 stone bridge and putting a telephone receiver in the place 

 of the galvanometer. By using an unusually sensitive 

 piece of selenium Ruhmer was able by such an arrangement 

 to send messages seven kilometers. 



However, this method has not been found to be a com- 

 mercial success. In the first place it can be used only 

 when the light of the arc shines on the receiving apparatus. 

 A fog, for example, would destroy its usefulness. Again 

 there can be no building or elevation obstructing the view 

 between the transmitter and the receiving apparatus. 

 Moreover the system can not be used in the daytime, for 

 changes in the intensity of the light due to any change in 

 the light of the arc are very small as compared with ordinary 

 -daylight, and finally under the most favorable conditions 

 the sound received is but a poor imitation of that spoken. 



Whistling Arc. The second device for wireless teleph- 

 ony has been somewhat more successful. It depends on 

 a discovery which appears to have been first made by 

 E. Thomson in 1892 l and later in a slightly modified form 

 by Duddell. 2 The principle of the discovery was the same 



1 U. S. Patent No. 500,630. 



2 Lond. Elec., 46, 310; 1900. Inst. Elec. Eng., 30, 232; 1900. 



