USE OF ARC IN WIRELESS TELEPHONY 



149 



in both cases, i.e., that when a condenser with self-induction 

 in series with it is shunted about an arc, oscillations may 

 be produced in the arc. The connections are shown in 

 Fig. 53 where A is the arc, C the condenser, 7 2 the induc- 

 tance in series with condenser, and K a key for opening or 

 closing this circuit. In general for the production of the 

 oscillations it is necessary to have an arc of high resist- 

 ance and a shunt circuit with low resistance. For example, 



AX 



FIG. 53. 



solid carbons must be used instead of cored carbons. The 

 nearer the arc is to extinction without being extinguished 

 the more apt it is to give the oscillations. With the appa- 

 ratus as shown by Duddell it was necessary to have less 

 than 2 ohms in the shunt circuit with about 5 millihenrys 

 inductance and a capacity of from i to 5 microfarads. 



This is due to the fact that when the current through the 

 arc increases, the potential difference needed to produce 

 it decreases. Thus, if the key at K, Fig. 53, is closed while 

 the current is flowing through the arc, the current will rush 

 into the condenser. If there is also inductance at /i, the 

 current flowing from the battery will not have time to 

 change more than a slight amount during the period that 

 the condenser is being charged. Under these circum- 

 stances the current which rushes into C will be taken 



