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The Ohio Journal of Science [Vol. XVI, No. 3, 



then at minute intervals and finally at much greater intervals. 

 When a new specimen is taken the initial currents are of the 

 order of several milliamperes for a few seconds. Such measure- 

 ments were made but it is difficult to obtain accurate current 

 measurements where the values diminish so rapidly. The 

 difficulty which Henderson and Weimar experienced in obtain- 

 ing definite break down readings are, I think, a little better 

 understood from the nature of these curves. In using a 60 

 cycle current a potential which increased from zero value to 

 maximum value in 1-240 of a second was applied; this decreased 



•so as 



Time i-n TYIiriutes 



to zero and was then reversed in direction. In such intervals 

 of time only a feeble back e. m. f. is developed and the current 

 no doubt has a large value. Their difficulty in building up a 

 definite potential was due to the fall in potential which occurs 

 when a current passes. The continued decrease in current 

 as time progresses is due undoubtedly to a back e. m. f. which 

 may attain any value short of the applied e. m. f. 



Persistence of the hack e. m. f. — A considerable part of the 

 experimental work in this connection was done in studying the 

 ability of these bodies to retain an e. m. f. set up in them. 



It was found for example that after the curve had been 

 followed out to the flat portion, that the e. m. f. might be 

 removed for a short time (say two minutes) and the curve 



