438 Prof. W. C. Rontgen on Electrical 



obvious that the line must be long enough to make an appre- 

 ciable difference between the sent and received currents. 



We may also determine k and i separately from the same 

 data. If I is the length of the line, then 



7 7 /T~' 1 v \ + 7i ^ 

 /•/= V /W log— — '* ) 



v = v ^i-i- log— - — — — — ■ 

 1 v 2 + 7 2 V fo 



> . . . (5) 



It is to be observed that these formulae give the true con- 

 duction and insulation resistances. The measured resistances, 

 or those deduced from observations with the bridge, differen- 

 tial galvanometer, &c, at the battery-end alone, are very dif- 

 ferent from the true, when the line is long and badly insulated. 

 The measured is always less than the true conduction resist- 

 ance, and the measured always greater than the true insulation 

 resistance ; while the measured conduction resistance can never 

 be greater than V ki, and the measured insulation resistance 

 never less. 



LVI. On Electrical Discharges in Insulators. By Dr. W. C. 

 Rontgen, Professor of Physics in the University of Stras- 

 burg* . 



IN the following communication are contained the results 

 of an experimental investigation begun long since, but 

 often interrupted, on the disruptive discharge of electricity 

 through insulators ; for I had set myself the task to discover 

 whether in such a discharge there exists any expressible rela- 

 tion between the physical constitution of the insulator, the 

 difference of potential required for a discharge, and the quan- 

 tity of electricity discharged. The investigation extended to 

 solid, liquid, and gaseous bodies ; but up to the present time 

 I have only succeeded with the latter in finding such a relation. 

 The solid bodies, mostly crystals, were placed, in the form 

 of thin plates, between two rounded -off brass points, one of 

 which was led away to earth, the other connected with a source 

 of electricitvj mostly a Holtz machine. By slow rotation of 

 the machine the potential was raised until a spark passed 

 through the thin plate. An electrometer specially constructed 

 for the present case permitted the course of the potential to be 



* Translated from a separate impression, communicated by the Author, 

 from the Nachfichten tier Kon. Geselhchaft zu Gottingen, 1878. 



