78 
PROFESSOR R, THRELFALL AND MR. J. H. D. BREARLEY 
criticise the de Sauty method of comparing capacities as applied to two condensers, 
one showing absorption and the other not (for sulphur is almost free from this 
phenomenon), but the largeness of our available voltage and the extreme perfection 
of our galvanometer permitted us to experiment under very advantageous conditions. 
For instance, with 100,000 ohms out in one branch, and about 7000 in the other, an 
appreciable kick was produced when the balance was upset by 1 ohm. Of course, it 
is necessary to use a thin film of sulphur in order to examine the resistance, and this 
is not the most satisfactory arrangement for arriving at the best value of the specific 
inductive capacity. There are two incidental advantages, however, which are of 
importance in practice. The first is that the correction for the effect of the edges of 
the plates is small. Thus, the actual area of the opposing plates was in one case 
found to be 156'445 square centims.; and with a film thickness of '0241886 centim., the 
correction for the edges (the plates being nearly square) amounted to '26 square centim. 
only, or about T4 per cent. As it was impossible to insure the plates being exactly 
over one another, and as any deviation from exactitude would diminish the capacity, 
and as, moreover, it was difficult to avoid the presence of small bubbles (whose 
area was approximately allowed for), it will be understood that the correction for the 
edges may be in general left on one side. It was, however, usually applied. Again, 
the capacity being tolerably large—between 2 X 1C -20 and 3 X 10 -20 C.G.S. electro¬ 
magnetic unit—the capacity of the connections and keys could also be neglected 
without prejudicing the results ; they were, however, included. The disturbance pro¬ 
duced in this case will be understood by the statement that the capacity of the keys 
and leads was measured by the ballistic method, as 'well as by the de Sauty method. 
The throw was 7‘8 divisions for a voltage which gave with the sulphur condenser in as 
well, 388 divisions. This corresponded to a capacity for the sulphur of '00226 micro¬ 
farad. Consequently the capacity of the leads and keys, &c., was about 2 per cent, 
of the capacity of the sulphur condenser. Now we shall show that the specific 
inductive capacity of prismatic sulphur is about 3‘7, and of the same sulphur when 
mixed with about '35 per cent, of insoluble sulphur is as much as 5'64 in one 
set of observations.* Now, whether our results are right or wrong, this shows that 
the correction of 2 per cent, is not of great importance. It can, howmver, be supplied 
if desired, as the key, leads, &c., remained the same throughout, and is supplied with 
the other corrections in the tables. 
The reader may possibly and naturally think that we ought to have observed the 
specific inductive capacity of sulphur in thick plates in order to get over the uncer¬ 
tainties mentioned, but it was one of our principal objects to discuss the specific 
inductive capacity as related to the conduction phenomena, and this could only be 
done with security by making both sets of observations on the same film. That this 
was a necessity, will be understood after an examination of the tables, which show 
* October, 1893.—We believe this liigk value is due to some error which, however, we cannot trace. 
The values of K are elaborately dealt with by the method of weighing later on, 
