﻿212 Mr. J. A. Brown on the Potential required 



of the field and definiteness of the spark-length in his dis- 

 cussion of Peace's results of the sparking potential. The 

 latter for pressures below the critical value could discover no 

 indications that Paschen's law held true; but the precautions 

 taken by Carr to eliminate the possibility of a brush-discharge 

 around the edges of the plates at low pressures, and other 

 disturbing influences, proved that the law extended to the 

 lower pressures as well. Such precautions were accordingly 

 also taken in the present work, in the hope that results would 

 be more definite than any other obtained previously. 



Apparatus. 



The spark-gap used was of the form suggested by Carr * in 

 his determination of sparking potentials. The only important 

 difference was that the walls were of glass instead of ebonite, 

 to enable one to watch the spark so as to determine whether 

 it filled the space entirely, and whether there was any flickering 

 or other unsteadiness. 



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The diagram needs no lengthy explanation. The electrodes 

 A and B were accurately turned, three pairs in all being used, 

 of zinc, aluminium, and silver respectively, the latter two 

 being set into brass to enable the tubes a a to be more easily 

 soldered on and to ensure a solid and air-tight plate. 



The spark-chambers were connected to one another in 



* Carr, I. c. p. 406. 



