558 
Proceedings of the Royal Society 
Ball and Plate. 
Length of Spark. 
Electromotive 
force for Positive 
Spark (1). 
Electromotive 
force for Negative 
Spark (2). 
Patio of (1) 
to (2). 
£ inch 
118'8 
129*7 
•92 
\ inch 
179-6 
201-7 
•89 
| inch 
219-2 
227-3 
•96 
1 inch 
234-6 
234-3 
1-00 
Thus under the above conditions and for a range of spark up to 
•5 inch, at least, the electromotive force required to produce the dis- 
charge is less when the ball is charged positively than when charged 
negatively. Within that range the discharge took place in the 
form of a single loud white spark, the index gave only one reading, 
and fell back after the passage of the spark almost to its ultimate 
position. But when the distance between the extremity of the ball 
and the plate was increased to *75 inch, the charge being negative, 
hissing sparks, giving only very small discharges as indicated by the 
behaviour of the index, preceded the loud spark which gave com- 
plete discharge. When the charge was changed to positive, the 
distance remaining the same, no hissing discharges were observed 
preceding the loud discharge. This is an instance of the pheno- 
menon to which Paraday refers, viz., that when the charged ball 
(under the above conditions) is positive, a longer spark can be 
obtained than when the charged ball is negative. When the 
distance was increased to 1 inch, hissing discharges preceded the 
loud discharge in both cases, but they were much more numerous 
in the case of the negative than of the positive charge. 
The results of the observations appear to explain this pheno- 
menon. They show that a charge of positive electricity requires a 
less electromotive force than a charge of negative, in order to force 
its way in the form of a spark (which is a complete discharge). A 
charge of positive electricity will therefore be able to discharge 
all together at a greater distance, provided we assume that the 
brushy spark begins at the same electromotive force for each. 
