of Edinburgh, Session 1879-80. 
555 
4. Positive and Negative Electric Discharge between a Point 
and a Plate and between a Ball and a Plate. By 
Alexander Macfarlane, M.A., D.Sc., E.R.S.E. 
I have made the following observations in the Natural Philosophy 
classroom of the United College, St Andrews, with the view of 
ascertaining whether the electromotive force required to cause a 
spark to pass between a small globe and a plate, or between a point 
and a plate, differs for the two kinds of electricity. Sir ’William 
Thomson suggested that I should apply to this question the method 
of measuring the electromotive force required to produce sparks, 
which I have described in papers already contributed to the Society 
(Trans. Roy. Soc. Edin. vol. xxviii. p. 633). It is a problem to which 
Earaday attached great importance. He says at sect. 1523, vol. 1, of 
his Experimental Researches in Electricity : “ The results connected 
with the different conditions of positive and negative discharge will 
have a far greater influence on the philosophy of electrical science 
than we at present imagine, especially if, as I believe, they depend 
on the peculiarity and degree of polarised condition which the 
molecules of the dielectrics concerned acquire.” He records a great 
number of experiments on this subject in sections 1465-1525. He 
took sparks between a ball 025 inch in diameter and a ball 2 inches 
in diameter. When the large one was connected with a discharging 
train, the small one charged positively gave a much longer spark 
than when charged negatively ; also the small ball charged nega- 
tively gave a brush more readily than when charged positively in 
relation to the effect produced by increasing the distance between 
the two balls (sect. 1489). When the interval was below 0*4 of 
an inch, so that the small ball gave sparks whether positive or 
negative, he could not, he says, observe any constant difference either 
in their ready occurrence or the number which passed in a given 
time. But when the interval was such that the small ball when 
negative gave a brush, then the discharges from it as separate 
negative brushes were far more numerous than the corresponding 
discharges from it when rendered positive, whether those positive 
discharges were as sparks or brushes (sect. 1490).* As he puts 
* Drs De La Rue and Muller have found in the case of the discharge 
of their great chloride of silver battery that the discharge between a point 
