248 ELEMENTARY LESSONS ON [CHAP. iv. 



The discharges in vacuum tubes are affected by the 

 magnet at all degrees of exhaustion, behaving like flexible 

 conductors. Under certain conditions also, the dis- 

 charge is sensitive to the presence of a conductor on the 

 exterior of the tube, retreating from the side where it is 

 touched. This sensitive state appears to be due to a 

 periodic intermittence in the discharge ; an intermittence 

 or partial intermittence in the flow would also probably 

 account for the production of striae. 



295. Electric Oscillations. Feddersen examined 

 the spark of a Leyden jar by means of a rotating mirror, 

 and found that instead of being a single instantaneous 

 discharge, it exhibited l certain definite fluctuations. 

 With very small resistances in the circuit, there was a true 

 oscillation of the electricity backward and forward for 

 a brief time, these alternate partial discharges being 

 probably due to the self-induction of the circuit. With 

 a certain higher resistance the discharge became con- 

 tinuous but not instantaneous. With a still higher 

 resistance, the discharge consisted of a series of partial 

 intermittent discharges, following one another in the 

 same direction. Such sparks when viewed in the rotating 

 mirror showed a series of separate images at nearly 

 equal distances apart. The period of the oscillations 

 was found to be proportional to the square root of the 

 capacity of the condenser. 



296. Velocity of Propagation of Discharge. 

 The earliest use of the rotating mirror to analyse phe- 

 nomena of short duration was made by Wheatstone, 

 who attempted by this means to measure " the velocity 

 of electricity " in conducting wires. What he succeeded 

 in measuring was not, however, the velocity of electricity, 

 but the time taken by a certain quantity of electricity 

 to flow through a conductor of considerable resistance 

 and capacity. Viewed in a rotating mirror, a spark of 



1 This phenomenon of oscillation was predicted from purely theoretical con- 

 siderations, arising out of the equations of self-induction, by Sir W. Thomson. 



