644 VELOCITY OF THE DISCHARGE. 



it close to some very fine iron filings, all possibility' of deception 

 will be avoided. For this experiment it is quite sufficient if the jar 

 has been charged by an electrophorus. 



The velocity with which electricity passes through 

 good conductors is astoundingly great, and consequently 

 the time required for an electrical discharge is exceed- 

 ingly small. Only by means of specially devised and 

 complicated apparatus is it possible to measure the in- 

 definitely small intervals of time which are the subjects 

 of observation in these experiments, and for their de- 

 scription the student must consult other works. As one 

 of the results of these experiments, it may be stated that 

 the electricity of a Le} r den jar passed through a wire 

 380 m long in 0-0000000868 second; this gives for the 

 space passed over in one second the enormous velocity of 

 nearly 440 millions of metres or about 280,000 English 

 miles, which is greater than that of light. It appears, 

 however, that there is no such .thing as a definite velo- 

 city of electricity. The time which elapses before an 

 electrical change produced at one end of a conductor 

 becomes perceptible at the other end, depends not only 

 on the nature and dimensions of the conductor, but also 

 on the nature and position of neighbouring bodies. 



The duration of the spark has been found to be not 

 greater than 0' 000 14 second, and not less than 0*00002 

 second. That the duration is extremely short may be 

 proved by a very simple experiment. The disc shown 

 in fig. 296 is fixed to the plate of the whirling table, 

 which is turned by an assistant as fast as can be done, 

 the room in which the experiment is- made being as 

 nearly dark as possible. The disc is then illuminated 

 for an instant by a spark, the jar being held with the 



