286 Prof. J. Trowbridge on 



same phenomenon could be observed in the ordinary voltaic 

 arc if it were not so exceedingly brilliant. 



I have lately studied the apparent resistance of the voltaic 

 arc in the following manner. In the circuit B (fig. 1) of forty 



Fis?. 1. 



large storage-coils giving 80 volts, was placed a low-resistance 

 choking-coil L, or coil of large self-induction. To the carbon 

 terminals A between which the voltaic arc was produced were 

 led terminals of a condenser C. The latter was charged by a 

 step-up transformer T. The oscillatory discharge of the 

 condenser was thus passed through the voltaic arc : and a 

 spark in a gap in the circuit of the condenser was photo- 

 graphed by the aid of a revolving mirror. The photograph 

 gave the number of oscillations in the circuit containing the 

 arc and the condenser. A curve was then plotted with the 

 number of oscillations as ordinates and the ohmic resistance 

 of the circuit as abscissas. It was thus found that the 

 apparent resistance of the voltaic arc was equivalent, in the 

 case I considered, to a resistance of eight tenths of an ohm 

 (*8 ohm). It was found, moreover, that an arc one quarter 

 of an inch long did not give more resistance than one one 

 half an inch long. The apparent resistance, therefore, of the 

 voltaic arc does not follow Ohm's law. I am led to believe 

 that the mechanism, so to speak, of the voltaic arc is as 

 follows : — A disruptive discharge accompanies a flaming dis- 

 charge, and serves as a species of pilot-spark. A variable 



