1892.] to produce a Spark in Air at different Pressures. 



FIG. 1. 



101 



To the Volt met er 



To the Cells- 



The path from the bell- jar to the pump passed through sulphuric 

 acid, and was also connected to a mercury gauge, Gr. Air could be 

 admitted to the jar through the stop-cock K, after passing through 

 cotton wool and sulphuric acid. 



The following was the usual course of a series of observations : 

 While the cells were charging, which generally occupied about one 

 half hour, the plates were polished and fixed in position, the cover 

 fitted to the mouth of the bell-jar, and the pump put in action. The 

 screw reading for contact of the plates was taken, the upper plate 

 being moved downwards till contact took place and completed the 

 telephone circuit. The plates were set at any required distance 

 apart, the upper plate being, in all cases, moved downwards to its 

 final position. 



When the cells were charged they were allowed a few minutes to 

 reach a steady state and were then joined up in series, and various 

 groups of 500 tested by the electrostatic voltmeter ; in this, the smallest 

 weight being used, one scale division read 50 volts, so that direct 

 reading gave ten times the voltage of a single cell. This measure- 

 ment was repeated at intervals during each experiment, and only 

 those observations were preserved in which the cells either remained 

 practically constant, or fell in potential difference so slowly and 

 uniformly that accurate allowance could be made. 



The plates being now at a known distance and the air at a known 

 pressure, regulated by the pump on the one hand and the stop-cock 

 K on the other, the number of cells connected to the plates, always at 

 first well under what was required to produce discharge, was gradu- 

 ally increased until the spark passed, and the potential difference 



