464 



Professor Arthur Schuster 



[Feb. 22, 



plate C, the electroscope is instantaneously discharged. The experi- 

 ment succeeds when a plug of cotton-wool is inserted at W to stop 

 the action of the dust ; but a plug of cotton-wool at the other end 

 diminishes the action so much that I am doubtful whether the effect 

 then really exists there. I am, so far, Dot inclined to believe that 

 the action is due to dust, but rather that the cotton-wool acts in 

 increasing very considerably the interval which elapses between the 

 time at which the spark acts and the time at which the sparked air 



Earth. 



Fig. 2. 



passes out of the tube. The effect may be observed even though the 

 tube L is lengthened by an addition of another piece 3 ft. or 4 ft. 

 long. 



Several phenomena, one of which had been known for a long 

 time, can be explained by the fact that the electric discharge changes 

 the condition of the gas into a state similar to that of gases rising 

 irom flames. It is mentioned, for instance, by Faraday that electric 

 sparks are liable to succeed each other along the same path, and it is 

 known that the same holds for lightning flashes, facts which them- 

 selves point to a higher conductivity of air along the path of the 

 previous discharge. A curious instance of a similar effect is afforded 

 by lightning conductors, which are sometimes put up to protect over- 

 head leads used for conveying a high-tension current. Owing to the 

 obvious impossibility of connecting the leads directly to earth, a 

 small air gap is interposed, the idea being that the air gap will act 

 as an insulator for the current the leads are intended to carry, but 

 that if during a thunderstorm the potential rises sufficiently high to 



