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PROFESSOR R, THRELFALL AND MR. J. H. D. BREARLEY 
pillars of ebonite of from 8 to 20 centims. long, and the two points of the reversing 
key requiring special insulation are made by placing ebonite cups on the ends of two 
pillars of fused quartz, each about 10 centims. long. The advantage of fused quartz 
as an insulator—where it is exposed to dusty air—-lies not so much in its great 
freedom from surface effect, as observed by Boys (‘ Phil. Mag.’ [5], vol. 28, p. 14), as in 
the fact that a small oxy-hvdrogen flame can be applied to it without fear of breakage, 
and thus the insulation be made practically perfect for the day, without any rubbing 
or deranging of contacts. The advantages of the general arrangements are— 
(1.) The points at which the insulation requires to be perfect are reduced to the 
fewest possible. 
(2.) The system being definitely earthed at the galvanometer, gives a complete 
protection against electrostatic effects, which are very noticeable when this precaution 
is not taken. 
(3.) The general insulation of the system is necessary when the evaluation of the 
galvanometer sensitiveness, or the capacity of the sulphur condenser, has to be 
determined, and it is a great convenience to secure this by disconnecting a single 
wire. The insulation of the “new galvanometer,” by means of fused quartz rods, 
will be explained in its proper place (see p. 88). 
The necessity for insulating the lower sulphur plate was well shown on one 
occasion, when it was omitted, to give rise to some trouble. Though this non-insulation 
produces no effect in so far as the applied voltage goes, yet it has practically the effect 
of short-circuiting the galvanometer through earth in one position of the reversing 
key and insulating one terminal of the galvanometer in the other. There are always 
minute voltages in the circuit, and these will cause a current through the galvanometer 
when it is short circuited. Hence, on reversing the key a throw will be observed 
which might be mistaken for a conduction throw due to conduction by the sulphur. 
As these voltages are very small, however, a sufficient insulation is easily obtained 
by a system of ebonite, paraffin and glass. 
Section IV. 
On the Construction of Sensitive High Resistance Galvanometers. 
In a paper, “ On the Measurement of High Specific Besistances ” (‘ Phil. Mag/ [5], 
vol. 28, p. 466), one of us laid down what he considered to be the most important 
direction in which to look for improvement in galvanometers. Since then work at 
the subject has been going on, though with many interruptions, both in the workshop 
and laboratory. During the time that has elapsed, the matter has received a good 
deal of discussion, notably from Professor Ayrton, and Messrs. Mather and 
Sumpner (‘Phil. Mag/ [5], vol. 30, p. 58). 
As to the corrugations on paraffined ebonite pillars, our experience is that they are 
