80 THE THEORY OF IONIZATION or GASES 



but since the force is very small along the distance 

 b bi, the experiments show that for a layer of thick- 

 ness b the potential fall is practically the minimum 

 sparking potential, and the product pb may be taken 

 as constant. 



34. Sparking potential at atmospheric pressure for very 

 short distances between the electrodes. 



Some experiments made by Earhart 1 dealing with 

 very short spark gaps in air at atmospheric pressure 

 seem to be at variance with the accumulated evidence, 

 which shows that there is a minimum spark potential 

 below which it is impossible to obtain a discharge 

 through a gas. This minimum is about 340 volts for 

 air, but in some cases, when the electrodes were very close 

 together, it was found that a current passed between 

 them when the potential difference was as low as 30 volts. 

 It was at first supposed that the current passed through 

 the air between the electrodes, but recent experiments 

 made by Almy 2 show that the electrodes are liable to be 

 drawn together and to come into contact, owing to the large 

 electrostatic force between them when they are a short 

 distance apart and a potential difference is established 

 between them. It was found that when the apparatus 

 was constructed so that the electrodes do not become 

 displaced a potential of 330 volts is not sufficient to 

 produce a discharge through air at atmospheric pressure, 

 while 360 volts is sufficient to do so. This result is in 

 accordance with the theory that has been given for the 

 sparking potential, and establishes the interesting fact 



1 Earhart, Philosophical Magazine, 1901, p. 147. 



2 J. E. Aliny, Philosophical Magazine, September, 1908. 



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