244 Behaviour of Air Sfc. under Powerful Electric Stress. 



and electromotive force departs from a straight line beyond 

 one million two hundred thousand volts, and approaches the 

 axis expressing the voltage. Thus the extreme length of 

 spark in air which I have been able to obtain with three 

 million volts is six feet and a half ; whereas a length of at 

 least ten feet should have been attained if the proportionality 

 between spark-length and voltage had been maintained. 



This departure from proportionality is due to the increased 

 conducting-power of the air ; for a powerful brush-discharge 

 is seen to proceed from the terminals of the apparatus to the 

 floor and the walls of the room. Hoping to diminish this 

 loss, I raised the apparatus three feet above the floor and 

 removed all metallic masses and pipes from its neighbourhood. 

 There was a slight gain in length of spark ; but it was evident 

 that the air yielded with great readiness to the powerful 

 electric stress. When a discharge occurred between the 

 spark-terminals a portion of it was shunted, so to speak, 

 through the surrounding air. The spark preferred to leap 

 through three or four inches of air to passing through one 

 thousand ohms of sulphate of copper between terminals of 

 copper one square centimetre in area It is probable that 

 with still higher voltage the initial resistance of air would 

 still further diminish, and would be of the order of metals. 



The initial resistance, too, of highly rarefied media dimi- 

 nishes in a similar manner. Thus a Crookes tube which 

 resists the passage of an eight-inch spark is brilliantly lighted 

 by a difference of potential of three million volts, and one 

 discharge of the duration of a millionth of a second is sufficient 

 to obtain a photograph of the bones of the hand. 



In a previous paper * I have spoken of the small resistance 

 of the electric spark, and of the singular fact that this resist- 

 ance does not increase materially with the length of the spark. 

 In approaching this subject from another point of view, I 

 was interested to note that Bjerknesf obtained a factor of 

 damping of 027 with sparks one millimetre long, and a factor 

 of 0*39 with sparks five millimetres long. 



Wt 



Since the factor of damping y= -^, in which W repre- 



sents resistance, t time, and L self-induction, it is evident 

 that the resistance W does not increase proportionately to the 

 length of spark. Bjerknes states that his experimental results 

 with long sparks were uncertain. This uncertainty, I am 

 inclined to believe, was due to the rapidly changing electro- 

 static field ; and to its wide extension when the spark-ter- 



* Phil. Mag. May 1897. 



t Wied. Ann. vol. xliv. p. 74 (1891). 



