AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
151 
The deflection due to direct action of the electromagnetic solenoid and included 
core when the galvanometer was disconnected from the “ bridge ” = -(-2 , 9. 
Therefore the deflection due to alteration of resistance = fl- 5 ' 0 . 
Immediately after taking the above readings the effect of altering by T ohm the side 
of the “ bridge ” adjacent to the side containing the bar was found to be a deflection 
of 190 divisions on the scale, and since the ratio of the resistance of the bar and its 
12-2 
copper terminals to that of the comparison piece was —-, it was assumed that the 
5'0 x 7 
increase of resistance amounted to J qq 12 -2 P er un ^- deflection of the needle of 
the tangent galvanometer was in this case 4°, and a similar set of observations was 
made when the M.C. produced deflections of 6 - 2 5°, 8'2 5°, 1075°, and 13°, with the 
results recorded below. 
Table XXXL 
Deflection of tangent 
galvanometer = <p. 
Tan <p + ‘00 8i<p sec <p — c. 
Increase of resistance 
due to magnetization in 
terms of divisions of the 
galvanometer scale = a. 
a 
c 
a 
C 2 ' 
4-00 
•104 
5V 
484 
462 
6-25 
•163 
9-8 
60-1 
369 
8-25 
■215 
17-0 
797 
368 
1075 
•284 
24-0 
84-5 
298 
13-00 
•343 
32-5 
94-4 
275 
It would seem that the increase of resistance produced by the magnetization varies 
in this case more nearly as c 3 than as c ; if, however, we take the mean value of the 
numbers for - we shall probably obtain a sufficiently close approximation to what can 
only be regarded as a rough measurement of the effect of magnetization on the resis¬ 
tance. This mean value is 73’2. The resistance of the nickel only was to that of the 
nickel and the connexions in the ratio of 3*8 to 12‘2, and since the value of c must 
be multiplied by ’316 in order to obtain the value of the current in C.G.S. units, 
and since moreover a unit current would produce an average magnetizing force in this 
814 
case of -yy X '7 X 47r, we see that the increase per unit of magnetizing force would on 
73 - 2 x'lx97 
the whole be ————— 7 ————-—— , or 4343 X 10 -8 . The bar was cast, and used 
•316 x 3'8 x 190 x 814 x 7 x 4ir 
in the same state as sent by the makers ;* when annealed, the value given above 
would be considerably greater, as it was afterwards ascertained that annealing very 
largely increased the capacity for induction from moderate magnetizing forces. 
* For this bar and for the bars of cobalt and bismuth used in the next experiments I am indebted to 
Messrs. Johnson, Matthey, and Co. All the bars here mentioned were very nearly chemically pure. 
