442 Prof.W. C. Rontgen on Electrical 



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intervals of time, gave values which agreed excellently with 

 each other. 1 resolved on this account, at least preliminarily, 

 to direct my attention chiefly to the determination of this dif- 

 ference of potential, which we will name the minimum poten- 

 tial-difference, and, for brevity, denote by M. P. 



The moment when the discharge ceases is, for the most 

 part, characterized by this — that the already much diminished 

 galvanometer-deflection (amounting to only 2-4 scale divi- 

 sions), after a further very slight lessening of the resistance in 

 the rheostat, suddenly becomes zero. At this instant the M. P. 

 is read off at the electrometer. I am inclined to account for 

 this phenomenon by the small variations which the potential 

 undergoes notwithstanding the insertion of the Leyden jars. 

 The electrometer, which is provided with a powerful damper, 

 gives the mean value of the variations of the potential. The 

 fact that the discharge had now really ceased I verified also 

 in another way : that is to say, if the galvanometer was made 

 considerably more sensitive by being rendered more perfectly 

 astatic, its deflection vanished at exactly the same difference 

 of potential as before ; in like manner an electroscope, which 

 instead of the galvanometer was connected with the plate in 

 the discharge-apparatus, was not charged, and the characteristic 

 star-shaped luminous appearance visible in the dark, which 

 was present during the discharge, disappeared when the M.P. 

 was attained. 



In all the following experiments the distance of the point 

 from the plate remained the same. Further, the temperature 

 was constant, at least in those experiments which were to be 

 compared with one another; and, lastly, it is to be noticed 

 that the point was always positive when the contrary is not 

 expressly stated. 



Unfortunately, the investigation had to be interrupted, first 

 because the seasons of spring and summer are very unsuitable 

 for working with static electricity, and secondly because for 

 its continuation the reconstruction of some of the apparatus, 

 especially of the electrometer, had become absolutely neces- 

 sary. Consequently, of the many questions which might be 

 put, only a few can be answered. The results are given below. 



1. How does the M. P. in a gas depend on the pressure? 

 The question was repeatedly answered for dry air free from 

 carbonic acid. Fig. 1 represents the result of one experiment: 

 the pressures in millims. of mercury were laid down as ab- 

 scissas, and the M. P. as ordinates. The unit in which the 

 latter are expressed is not directly comparable with that men- 

 tioned above. 



