288 



Dr. Norman Campbell : Experiments on 



the connexion between the source of potential and a volt- 

 meter reading such potentials directly. For if the break 

 occurred when the potential was rising, the subsequent 

 increase of potential would be sufficient to cause a spark to 

 pass across the gap as the terminals separated. Even if 

 this difficulty could be surmounted, others obviously attending 

 the breaking of a circuit currying such high voltages would 

 bo encountered. Accordingly the device was adopted of 

 reducing the potentials to be measured in a known ratio 

 by means of condensers connected in series. The arrange- 

 ment is shown in fig. 1, where it is supposed that the 

 potential developed in the secondary circuit is being- 

 measured. 



1. 



P is the primary circuit, across the terminals of which is 

 connected the condenser 1? supplied with current from the 

 battery B controlled by the rheostat R. The primary 

 current is broken by the opening of the contact 1 by the 

 pendulum. (It should be observed that the primary con- 

 denser is connected across the primary coil and not across the 

 contact ; the battery and rheostat do not form part of the 

 oscillating circuit.) The high-potential terminal of the 

 secondary is connected to one plate of the air-condenser (J 2 

 of about 10 mmf. capacity*; the other plate is connected 

 to one side or! condenser C 3 ', the other side of which is 

 earthed, and, through the contact 2, to the parallel condenser 

 C 3 , which again is joined through the key '6 to the Dolezalek 

 electrometer D. The other plate of C 3 is connected to earth 

 through the potentiometer arrangement shown, by means of 

 which any desired potential can be established between the 

 second plate of C 3 and earth. The contact 2 is the second 

 contact broken by the pendulum. 



* 1 mmf. = 10" 12 farad =0-9 cm. 



