76 
PROFESSOR R, THRELFALL AND MR. J. H. D. BREARLEY 
for qualitative rather than absolute quantitative results, and for this purpose his 
method has advantages in its quickness and simplicity. If absolute values are 
required, the method has many drawbacks. The artificial resistances are subject to 
considerable variation, and are themselves calibrated one from another in such a 
manner as to give every opportunity for the accumulation of errors. There is reason 
to suppose that the resistance to feeble currents, such as would be produced by a few 
cells, will depend on the previous history, and will not be by any means the same 
when the current is reversed. In Roller’s experiments the current was, in general, 
not reversed; in fact, the peculiar behaviour of the liquids investigated did not 
permit of the method of reversal being employed in all cases. Since the resistance 
comparison depends on the comparison of two potential differences, it is necessary to 
eliminate the electrode effects without taking advantage of the method of reversals. 
Roller allowed for the electrode effects by observing them when no current was 
flowing, which was, perhaps, as much as the circumstances permitted, but could 
hardly be considered sufficient when absolute measurements are in question. Roller 
does not appear to have reversed his electrometer {Joe. cit., p. 204) in taking his 
readings ; an oversight which, we are persuaded, must damage the authority of his 
work. We are, however, more concerned to explain why Roller’s apparently simple 
and advantageous method was not suitable for the ends we had in view, than to 
criticise his work. We may add that the power of the method is finally limited by 
the want of insulation of the electrometer, but it remains probably the most sensitive 
method known for the examination of high resistances. 
Method Adopted for Measuring High Specific Resistances and Specific Inductive 
Capacity. 
The method employed by us # is substantially the same as that explained by one of 
us and J. A. Pollock, in a paper in the ‘ Philosophical Magazine/ vol. 28, 18S9, p. 469. 
The sulphur forms a film between two aluminium plates. A voltage up to about 
300 volts is supplied by a set of small test-tube accumulators. The sulphur, storage 
cells, and galvanometer, are placed in series, keys being provided for cutting the 
galvanometer out of the circuit, and for reversing the current through the galvano¬ 
meter without reversing it in the rest of the circuit. Readings are always taken by 
the method of reversing the current through the galvanometer, and observing the first 
elongation of the needle. Two galvanometers are always in commission—one at a 
very high grade of sensitiveness known as the “ new galvanometer” and the other 
at a lower grade, read by a spot of light on a scale, known as the “ old galvano¬ 
meter.” In order to evaluate the readings of the galvanometers, a second set of 
apparatus is prepared, consisting of a standard Clark cell, a megohm, and various 
resistance boxes, whereby any voltage, from that of the cell down to one hundred- 
* See Plates 2-5. 
