290 Dr. Norman Campbell : Experiments on 



contact 3 insulating the electrometer from the rest of the 

 apparatus. The residual deflexion was then read and applied 

 as a correction to that given by the potentiometer. 



It will be observed that this method has the advantage 

 that the potential in the secondary can be observed without 

 adding to it any capacity greater than that of a small air- 

 condenser 2 . The capacity to earth actually added was 

 11*8 rnrnf.. and could have been made even smaller if it had 

 been desired. 



In order to indicate the accuracy obtainable with this 

 method of measurement, there is given in fig. 2 the wave- 

 form of the secondary of a magneto, all the experimental 

 points being shown. It will be seen that two of them arc 



Fii 



6000 



2 4000 



>5 



? 2000 



1000 





•0001 

 Time (seconds). 



clearly in error, but that the rest lie very closely on a 

 smooth curve and enable the relation between the secondary 

 potential and the time to be determined with an accuracy 

 amply sufficient for all the purposes of this investigation. 

 In general, since only the period of the oscillations was 

 required, the whole wave-form was not traced, but only the 

 part in the neighbourhood of the maxima and minima; such 

 observations permit the period and the logarithmic decrement 

 to be deduced. 



