q24 REPORT—1890, 
wax a large central mercury cup, and outside this a circular channel ; 
thick copper plates were cut to fit them, and both plates were well 
amalgamated. By means of this cup arrangement the three B.A. units 
(H., G., and Flat) were connected in multiple arc, and by means of 
stout copper rods the multiple-arc arrangement was connected with the 
mercury cups on the Fleming’s bridge, and so compared with the 3 
B.A. unit. The following observations were taken :— 
July 12, 1889: 1(18°-4) + 986-6(b.w.d.)=M.A. + 24-6(b.w.d.) 
July 22, 1889: 3(17°4) +986 (b.w.d.)=M.A. +241 (b.w.d.) 
August 26, 1890: 1(16°-8) +986-1(b.w.d.) =M.A. + 23-9(b.w.d.) 
The value of a bridge wire division (b.w.d.) is ‘0000498 B.A. unit at 
15°, and the wire has a temperature coefficient of -00143. 
It is evident from these series of values that the + has not changed 
in resistance during the period of the experiments. 
This comparison, however, introduced a possible error, as the tem- 
perature of the bridge wire at the time of experiment was not accurately 
known, and this is important when nearly the whole of the bridge wire 
is employed. To eliminate this possible error the + was compared 
with four B.A. units in multiple arc. In this case a large number of 
bridge wire divisions had to be subtracted from the value of the 3, and 
the whole number of bridge wire divisions entering into the calculation 
for the values of the 3 was largely reduced. The four coils in multiple 
arc were (IF, G, H, and Flat) :— 
Aug. 25, 1890: 3,16°8+4157 (b.w.d.)=M.A.+852:05 (b.w.d.). 
Aug. 26, 1890: 2, 16°8+157-5 (b.w.d.)=M.A. +851°9 (b.w.d.). 
All the four coils were at the same temperature (16°°8). Their values 
are taken from the * B.A. Report,’ 1888 :— 
Flat. ; é ; : . 1:000448 
F : : ; : : . 1:000028 
G 5 ; : : : ow ae 
is : : : : - + auoeoe 
They give for the two multiple-arc arrangements the values ‘33330 
and °24998. The connecting-rods have a resistance of ‘00042, and the 
value of the 4 at 16°8 is -28537 B.A. unit. Its temperature co- 
efficient is ‘0001 per 1° C. 
To measure the lengths of the wires two microscopes with scales and 
verniers reading to ‘] of a millimetre were set up and firmly clamped in 
position; the distance between them was determined by means of a 
beam compass and the aid of a third microscope: the distance between 
this and the other two being directly read off on the beam compass for 
set positions of the verniers. The wires were cut with a fine fret-saw 
at the points corresponding to the position of the riders in the resistance 
measurements. Before weighing the wires were carefully cleaned with 
methylated spirit. The balance employed was the one used by Mr. Glaze- 
brook for our determination of the specific resistance of mercury; the 
weights were balanced against one another, and in all cases double 
weighings were taken. 
The specific gravity of most of the wires was determined ; for this pur- 
pose distilled water was boiled and cooled rapidly, the coil of wire 
immersed, and the beaker and its contents placed under the receiver of 
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