618 Mr. G. Moody Hobbs on the Relation between P.D. 



large ranges of pressures and distance was investigated, and 

 several important laws discovered. The linear relation 

 between spark-potential' and distance for comparatively large 

 values of the latter was established, and Peace noticed the 

 existence of a minimum spark-potential. Paschen deduced 

 from a large number of observations the law which bears his 

 name, viz., that for given potential-differences the product of 

 the sparking distance and the maximum pressure for pro- 

 ducing a spark is a constant. However, it was not until 

 1900 that any work was done to prove the correctness of 

 these laws for very small distances. At this time, Earhart 

 carried on an investigation in this laboratory in which, by 

 the aid of the interferometer, he was enabled to make 

 accurate observations down to the point of contact. His 

 results were very interesting and, when plotted with potentials 



Yig-, 1. — In Air at Atmospheric Pressure. 



450 



400 



350 



| 30 



^Z50 

 ^ 200 



J 150 



100 



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10 15 



DISTANCE 



20 

 IN 



25 30 



WAI/E-LENGTHS. 



35 



40 



as ordinates and sparking distances as abscissae, gave in air 

 at atmospheric pressure a straight-line curve (see fig. 1) 



