1902.] on Musical and Talking Electric Arcs. 67 



light as variations in its intensity, and are faithfully reproduced in 

 the telephone receiver, conveying sounds and speech just as if con- 

 necting wires had been established between the two stations. 



Transmission of speech by this method has, I believe, been 

 already accomplished over distances of several kilometers by Herr 

 Ruhmer. 



A further development of the uses of the talking arc, originated by 

 Herr Ruhmer,* is the production of a new phonograph founded on 

 similar principles. A permanent record of the variations of the light 

 emitted is obtained by photographing on a long rapidly moving film 

 the light given out by a talking arc so as to obtain, after development, 

 a band of deposit, the changes of density along the length of which 

 are a record of the variations in the intensity of the light emitted by 

 the talking arc. If such a film be subsequently caused to travel at 

 the same speed as during the recording, and if a beam of light be 

 passed through it on to a selenium cell, the intensity of the light 

 beam and the illumination of the cell will vary, and speech will be 

 reproduced in a telephone receiver connected in series with the cell 

 as before. This new phonograph adds one more to the methods, 

 mechanical, magnetic, and chemical, by which sound waves can be 

 permanently recorded, and it should give us much information as to 

 the nature of sounds owing to the ease with which the record can be 

 visually examined. 



The second part of my subject treats of quite a different property 

 of the electric arc, a property by means of which a direct current can 

 be automatically converted into an alternating current of almost any 

 frequency. This property is the instability of the arc under certain 

 conditions. 



Before proceeding further I will recall the properties of the dis- 

 charge (or charge) of a condenser, or Ley den jar, through an ind:ic- 

 tive circuit. Lord Kelvin proved that if the resistance of the 

 inductive circuit be below a certain critical value (depending on the 

 capacity of the jar and self-induction of the circuit) then the discharge 

 current would not flow continuously round the circuit in one direc- 

 tion till the jar was discharged, but would first flow round the circuit 

 in one direction and then in the other, executing a series of swings or 

 oscillations like a pendulum, the maximum value of the current 

 growing less and less each swing, until the oscillations finally died 

 away and the jar was discharged. If the resistance of the circuit was 

 so small that it could be neglected, Lord Kelvin also showed that the 

 periodic time, or time taken for the current to rise from zero to a 

 maximum in one direction, then die away, reach a maximum in the 

 opposite direction and again come to zero, was given by the 

 expression 



Periodic time = ^tt J L.F. 



♦ Elektrotechnische Zeitsohrift, 1901, p. 198. 



