106 REPORT—1890. 
a2 
de ak “1 1- Geet GHt@ 
ae b a 
ay fey eS \ { 1. 2 
j acer @ Sry: + T@terg) 
In most of the experiments about to be described, we had the following 
values in legal ohms :— 
a=10 d= 1,000 
6=18 g = 17,600; 
while c, which was the adjustable arm, varied from 6,000 to 15,000. 
With these values, the only correction which need be included is the 
last factor in the denominator, and we may write 
a 
wy) 
ed] Be (at+e+g) J 
The resistances were taken from a legal ohm box belonging to the 
laboratory ; the various coils in this box were carefully compared with 
each other by Mr. Searle, and found to be consistent with each other, at 
any rate to within 1 in 10,000. The coils were also compared with the 
standards of the Association, and it was found that at 16° they were 
greater than legal ohms in the ratio of 1:0011 to 1. The standard tem- 
perature adopted in the experiments was 17°, and since the coefficient of 
increase of resistance of the box is about ‘0003 per 1° C., the resistances 
require to be multiplied by 1:0014, to reduce them to legal ohms. In 
some cases, in the value of c, coils from a B.A. unit box, containing coils 
of ten, twenty, thirty, and forty thousand, B.A. units were employed. 
The values found for these coils by myself in terms of the legal ohm 
box showed that they were very consistent with each other, and that the 
nominal 10,000 B.A.U. was equal to 9,880 legal ohms as measured by 
the legal ohm box. 
In the comparisons of two condensers certain coils from a megohm 
box were used; the value of each of these was also determined. They 
were as follows :— 
nC= 
1 nob 98,731 Legal ohms of standard box. 
Boe 98 65 rs ‘ 
3 ae 98,698 ‘i A 
Fea) 5S 735 if 
9 ae 98,725 iy x 
10 fe 98,776 "s Me 
In the experiments on Dr. Muirhead’s condensers, the vibrating com- 
mutator described in Professor Thomson’s paper, ‘ Phil. Trans.,’ 1883, or 
in my paper, ‘ Phil. Mag.,’ 1884, was used, and that with complete suc- 
cess. In the experiments about to be described, this was replaced by a 
rotating commutator which had been fitted up by Professor Thomson 
and Mr. Searle for their experiments on the other value of ‘v,’ and which 
possesses certain advantages over the other form. Dr. Muirhead and 
Dr. Fleming have also used a somewhat similar arrangement of appa- 
ratus. Fig. 5 shows the arrangement. The split ring commutator is 
carried on the axle Hk, which is driven by a water motor. Two wire 
springs, Q, R, are in contact with the two halves of the commutator 
respectively, and as it rotates the brush p, made of very fine brass wire, 
Gules. _- 
