Xfierheller] to determine MINIMUM SPARK POTENTIALS 



IDS 



In using this apparatus after the manner of the earlier workers 

 one of the electrodes c was joined to a quadrant electrometer or to an 

 electrostatic voltmeter, diagram I, Fig. II, also initially earthed through 

 a bye-pass key. The potential selected, which was obtained from a 



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i r 



(g)- 



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•storage battery protected with a water or xylol resistance in circuit, 

 was then applied to the electrode d and the earth connection to c broken. 

 If no discharge occurred after waiting some minutes, higher and higher 

 potentials were successively applied until discharge did occur. Gener- 

 ally this was indicated by a violent deflection of the electrometer or 

 electrostatic voltmeter needle. 



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By following this method a set of spark potentials was obtained 

 with the air at different pressures when the electrodes were 3.3 mm. 

 a])art. The values of these potentials together with their corresponding 

 pressures and a curve to illustrate them are shewn in Fig. III. The 

 lowest sparking potential, it will be seen, was obtained at a pressure of 

 .i^bout 2.3 mm. of mercury and amounted to about 388 volts. This poten- 

 /tial, (and in fact all the potentials for the different pressures investi- 



