146 
PROFESSOR R. THRELFALL AND MR. J. H. D. BREARLEY 
The attraction, without the sulphur, having been observed, the attraction is again 
measured with the coil commutator reversed, in case there is any outstanding 
electrification ; in general this produces no effect. There was no difference in any 
of the experiments leading to our values. 
The next step is to earth the high potential plate and carefully dust every part of 
the apparatus, including the sulphur plate ; the connecting wire is dusted by running 
a flame along it. The sulphur plate is then inserted, and an observation made of the 
value of the counterpoise, in order to find if there is any free electrification. If there 
is, the plate is taken out and tlie electrification removed by a flame—a risky pro¬ 
ceeding, and one we never indulged in unless it was absolutely necessary. The 
process of finding the balance was then repeated as before. When a sufficient 
number of observations, both with the plate in and out, had been made, the screens 
were removed, and the distance between the guard-ring and the glass plate 
remeasured. There was never a difference of more than T millim. At the end of 
the series of experiments all the plates available were experimented on in this manner 
one after the other, so as to get good comparative results. The apparatus was then 
dismounted, and the distance between the fixed and suspended plate changed, after 
which the adjustments were re-made and the whole series gone through again at the 
new distance, with a view to eliminating accidental errors of setting, weighing, and 
measuring. 
The sulphur plate must not be less than five or six millims. from the suspended 
plate, or the free electrification at its upper surface has a sufficient P.D. from that of 
the earth to cause brush discharges. Consequently tve always examined some plate 
or other in the dark to see if this did occur. If it does, of course, the result is that 
we get too low readings and a too low value of K. On the other hand, the nearer 
the top of the sulphur plate is to the suspended plate the better, for the greater the 
forces to be observed. The “ art ” of the experiment, we should say, lies in adjusting 
the voltage till brushing just does not occur. This is also provided for by placing the 
coil terminals so near together that a discharge occurs there before it occurs between 
the sulphur and the earthed part of the apparatus. It is also exceedingly necessary 
to remove all dust and sulphur crumbs between each set of weighings. 
On thinking the matter over in the light of our present experience, we have an 
idea that there might be advantages in using a liquid of high electric strength instead 
of air—in fact, to proceed as Quincke did in determining thfe dielectric constants of 
liquids by weighing, in order to get the advantage of the large forces he was able to 
measure. 
Analyses of the sulphur plates were made by breaking them up, and taking samples 
from the active part. This was also necessary in order to find out whether there were 
any cavities in the plates. In only one plate was a cavity discovered, but happily it 
was under the guard-ring almost entirely, and was very insignificant. 
