Discharge of Electricity. 



403 



a spherical ball 0*5 inch in diameter fixed on the point. A 

 circular plate, 7 inches in diameter and fixed on the other rod, 

 formed the uninsulated electrode. The sign of the electricity 

 was changed by merely charging the one or the other paper 

 conductor of the machine. The electrometer had either half- 

 ring capable of being insulated ; and I was thereby enabled to 

 take readings in both directions for each kind of spark, and so 

 eliminate any difference due to a bias in the instrument. 



With the point for charged electrode the following results 

 were obtained : — 



Difference of 

 Distance J potential for 

 between point positive 

 and plate. discharge. 



(i) 



Difference of 



potential for 



negative 



discharge. 



(2) 



Difference of 

 (1) from (2). 



-§ inch. 



1 „ 



2 „ 



76-6 



863 



102-3 



67-1 

 76-2 

 95-2 



9-5 



10-1 



7-1 



The behaviour of the index showed that the discharge was 

 not single, but a rapid succession of small discharges; for 

 it first attained a temporary maximum deflection, and then a 

 steady deflection slightly less than the maximum. The latter, 

 on account of its being capable of being observed with greater 

 precision, was the one recorded. This agrees with Prof. 

 Rontgen's* experience. It would have been better if a com- 

 parison had also been made at a distance where the discharge 

 could take the form of a spark ; but as it is, the observations 

 show that in the region of the brush or convective discharge 

 the curve for the point negative is similar to that for the point 

 positive, and slightly lower. Is this difference due to the elec- 

 trical state of the air in the neighbourhood of the point ? 



With the ball as charged electrode the following results were 

 obtained : — 



Length of 

 spark. 



Difference of 



potential for 



positive 



spark. 



(1) 



Difference of 



potential for 



negative 



spark. 



(2) 



Difference of 

 (1) from (2). 



\ inch. 

 * » 



1 M 



118-8 

 179-6 

 2192 

 234-6 



129-7 

 201-7 

 227-3 

 234-3 



-109 



-22-1 



- 8-1 



•3 



In the case of the first two distances the discharge took place 

 in the form of a single loud white spark, the index gave only 

 * Phil. Mag. Dec. 1878, p. 141. 



