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XII. On the Least Potential Difference Required to Produce Discharge through 



Various Gases. 



By the Hon. II. J. STRUTT, B.A., Scholar of Trinity College, Cambridge. 

 Communicated by Lord RAYLEIOH, F.R.S. 



Received October 17, Read November 16, 1899. 



Introduction. 



MANY experimenters have made observations on the potential difference necessary to 

 produce electric sparks through gases. The field is a very wide one, since the number 

 of circumstances which may be varied is large. The nature and the pressure of the 

 gas, the shape of the electrodes, the distance between them, and the pressure of the 

 gas, may each be altered. The investigation of which I wish to give an account in 

 this paper deals with the potential difference required to produce sparks (or striking 

 potential as I shall call it for brevity) in various gases, between large parallel planes, 

 at a fixed distance apart, and at various pressures. 



It was found by Mr. PEACE ('Roy. Soc. Proc.,' vol. 52, p. 99) that the striking 

 potential between two parallel plates in air diminished as the pressure diminished till a 

 certain point was reached, and then began to rise very rapidly. The pressure at which 

 the striking potential was a minimum depended on the distance between the plates, and 

 increased as the distance was lessened. The minimum potential itself, however, 

 varied very little with the distance between the plates. 



This minimum potential was of the same order as the cathode fall of potential in 

 air, as has been pointed out by Professor J. J. THOMSON ('Recent Researches in 

 Electricity and Magnetism,' p. 158). The following explanation may 1 offered of 

 the fact that this is a minimum striking potential, and that it is approximately 

 constant. 



The negative glow in any gas [as has been shown by WARBURG (' Wied. Ann.,' 31, 

 p. 579)] requires for its production a definite potential difference (about 340 volts in 

 the case of air), independent of the pressure, and constant, so long as the glow is not 

 crushed into a smaller space than that which it would naturally occupy. If the glow 

 is crushed, the potential fall is more. Let us now suppose that the discharge takes 

 place between two parallel plates ; a part of the space between these plates is 

 occupied l>y the negative glow, a part by the positive column. So long as any of the 



VOL. CXCHI. A. 3 C . 1.1900. 



