VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
235 
Value of Flat coil at 15° C. . 
Temperature of coefficient . 
Value of V at 15° C. 
Temperature of coefficient . 
1-00003 
•00028 per 1° C 
1-0015 
•0003 per 1° C. 
The coil itself is enclosed in a brass case and could be placed in a vessel of water. 
This was done during the experiment and the temperature noted by a thermometer 
graduated to fifths of a degree centigrade. 
The coil S used with the galvanometer, when part of the battery current was sent 
through it, was a coil of platinum-silver wire of the ordinary form of about 3000 units 
resistance. It was immersed in the same vessel of water as V, and its temperature 
read by the same thermometer. 
In our first series of observations the total value of the resistance M P Q G H, 
was observed and found to be 3072*38 ohms when the temperature of the coil S 
was 13*2. 
The value of V at this temperature is 1*0011 ohms. 
One extremity of the coil V dipped into the same mercury cup H as one extremity 
of the coil S, and the battery was also connected with this cup. The other extremity 
of V was connected by means of a piece of copper wire with L, the mercury cup in 
which the two portions of the battery current again united. This piece of copper 
wire was found to have a resistance of -01556 ohm, so that the value of the resistance 
in the circuit HVL is P0167 ohms, at a temperature of 13°"2, and the currents in 
the two branches HVL and II G M L respectively, will be in the ratio of 3072"38 
to 1 "0167, so that if i be the batterv current, that passing through the galvanometer 
will be 
T, as has been explained, was a variable resistance which could be adjusted so as to 
keep the difference between the resistance of the secondary circuit and the standard R 
sufficiently small to be measured in terms of part of the wire of the bridge E F. 
During the experiments T had to be varied by somewhat over half a B.A. unit. 
Now T enters with S into the galvanometer circuit. The resistance, therefore, of 
this circuit was not quite the same during the observations, but varied by somewhat 
over -25 ohm from its mean value, 3072*38 ohms. 
The resistances W and LT were two coils of about 30 ohms each wound on the same 
bobbin, and made of the same wire. 
The galvanometer used with the Wheatstone’s bridge was one of about 150 ohms 
resistance, made by Professor Stuart at the mechanical workshops, Cambridge. 
Theory of the experiments. 
Let R be the absolute resistance of the secondary circuit including the galvano¬ 
meter, M the coefficient of mutual induction between the coils, and i the current in 
