Perkins — Velocity of the Propagation of Magnetism. 167 



quadrants the other. By means of two Kempke discharge keys 

 the circuits were so arranged that the electrometer was nor- 

 mally on short circuit, but on pressing the keys simultaneously 

 the terminals of the external circuit were simultaneously attached 

 to the electrometer terminals. This method was essential, for 

 with one terminal free and the other connected to some metal 

 object the instrument would be most violently deflected ; hence 

 the necessity for making the contacts exactly together. 



As it was purposed to measure alternating voltages, the instru- 

 ment was calibrated with the use of alternating e.m.f. of the 







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same frequency. These were measured by " weighing" in a 

 Kelvin balance the current that went through a non-inductive 

 resistance across whose terminals the electrometer was connected. 

 The resistance was measured after each reading to allow for 

 possible changes due to heating. It may be of interest to note 

 that the calibration curve as obtained by this method differed 

 considerably from one obtained with constant voltages. This 

 was doubtless due to the capacity introduced by the cup of 

 sulphuric acid into which the needle dipped. 



Several readings were necessary in determining a single angle. 

 Calling the small central coil wound on the exciter " A," and 

 the more distant coil " B," we may simplify the description as 

 follows : The electrometer was flrst connected to A, giving a 

 deflection D a which corresponded to an induced potential V a . 



