the High- Tension Magneto. 289 



If V 2 is the potential difference between the high-potential 

 terminal of the secondary and earth at any moment after the 

 opening; of 1, then v, the potential difference across C 3 or 

 C 3 ' at the same moment is given by 



e 2 (v 2 -^-(c 3 +c 3 > (5) 



If contact 2 is opened at this moment, contact 3 being closed, 

 the electrometer will finally register the potential v. Since 

 it is convenient to use the electrometer as a null instrument, 

 the needle of the electrometer is brought back to zero by 

 applying to the second plate of C 3 a potential difference 

 «/= — v by means of the potentiometer; V 2 is determined by 

 the value of v' necessary to bring the needle back to zero. 

 If the capacity of the electrometer itself and of the various 

 leads is taken into account, the relation between V 2 and v' is 

 rather more complicated than that stated ; but it is easy to 

 see that the ratio V 2 /v' will be constant, so that it may readily 

 be determined by calibration with a known V 2 . 



The object of the condenser C 3 should be explained. If it 

 were absent, then after contact 2 is opened, the second plate 

 of C 2 would ha v e a very small capacity to earth, and its 

 potential would vary nearly as rapidly as that of the first 

 plate. A potential of several thousand volts might be 

 established across contact 2 and a spark would pass. CV, com- 

 posed of le}rden-jars, was so chosen that the potential across 

 2 could never rise above about 30 volts. C 3 , composed 

 chiefly of good mica condensers, was chosen so that v' was of 

 the order of 2 volts, which could be measured with an 

 accuracy of 1 in 500*. 



In order to minimize insulation losses when the greatest 

 accuracy was desired, other contacts were worked auto- 

 matically by the pendulum. The sequence was then as 

 follows. The pendulum first opened a contact disconnecting 

 the electrometer from earth ; it then operated contacts 1 

 and 2 ; it next closed the battery circuit of the potentio- 

 meter, thus applying to C 3 a potential which would very 

 nearly bring the needle back to zero, and lastly broke 



* C 3 included also the leads from the pendulum to the electrometer, 

 which were some yards in length. For these leads rubber cable, such 

 sis is used for connecting a magneto to the sparking-plug, was employed, 

 the outside of the cable being coated with an earthed conductor. Such 

 cable has an insulation resistance of more than 10 15 ohms per yard, and 

 is extremely convenient where very high insulation is required ; it may 

 be used for all but the most delicate electrostatic experiments to replace 

 the usual arrangement of bare wires supported on sulphur or sealing-wax 

 supports. It has the drawback of considerable dielectric hysteresis, but 

 it is often possible to arrange matters so that this is of no consequence. 



