ON THE ELECTRICAL PROPERTIES OE PURE SULPHUR, 
141 
remains entirely independent of the absolute value of anything (as a measurement of 
K, of course, should do); all that is required is a fairly accurate knowledge of the 
ratios of the weights employed, and of two distances which can be measured with 
fair accuracy. The drawback to the method is that it is not as susceptible of being 
carried to a high degree of accuracy as some more directly electrical methods. 
Both measurements of length were reduced to the same standard, by fine callipers of 
Elliott Bros., whose absolute indications are immaterial, but which we believe from 
other comparisons to be practically correct. 
The comparison of the gilded brass and platinum weights with each other was 
carefully carried out on a fine balance, and resulted in showing that the weights 
were so well adjusted that none of the corrections observed can affect our results—- 
the differences being much less than the error of weighing against the electric forces. 
As we spent several months in perfecting our apparatus, after the experiments of 
Mr. Allen and one of us were finished, we decided to illustrate our description with 
a small drawing to scale (Plate 5), which we hope may be of assistance to other 
people using the method, and which will enable us to shorten our description 
considerably. 
A is the zinc suspended disc. The lower surface was scraped true to a surface 
plate after the three pillars Y, Y, Y had been soldered to its upper surface. The 
diameter of this plate was 12 65 centims., and it weighed (uncorrected) 154'3002 
grams. 
Each of the pillars Y, Y, Y were bored and tapped so as to carry three screws— 
lying in a plane parallel to the plate. These screws are necessary to allow of the 
adjustment of the three fine German-silver wires, by which the plate is supported. 
It is necessary to have these screws well made if accurate and convenient adjustment 
is sought after. The point of junction of the three wires is attached to a single fine 
German-silver wire, which passes to below the balance case, where it is hung from a 
frame originally made for the purpose of weighing quartz rods in water, in order to 
determine their coefficient of expansion. This frame could be arrested just like the 
pan of an ordinary balance. The rough adjustment of the length of the wire was 
made by a minute box-wood shackle. The balance stood on a state shelf, and the 
rest of the apparatus on a slate table below it, and we may here remark that one of 
the first desiderata is steadiness and solidity in all the parts of the apparatus. A 
fine spiral of wire connects the plate to the guard-ring. 
B is a guard-ring of zinc, the top and bottom surfaces being turned parallel to each 
other in a lathe. The inner diameter is 13 centims. and the outer diameter 
23‘6 centims., leaving the ring with a radial breadth of 5’3 centims. The clearance 
on each side of the suspended plate is, therefore, ‘175 centim. We could have done 
with less clearance than this, but an examination of the theory showed that no error 
at all comparable with those of measuring and weighing would be introduced by an 
annular space of these dimensions. 
