162 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
would cause any variation in the resistance of iron wire, of which the next experiment 
will furnish a sample. 
Experiment LXXVII. 
An annealed iron wire, 14 inches long and '85 millirn. in diameter, was balanced 
against a platinum wire yg-th of an inch in diameter, and of nearly equal resistance. 
The B.C. was varied by interposing different amounts of resistance in the external 
circuit of one Grove’s cell, and this could be done very quickly by a suitable arrange¬ 
ment of mercury cups. Resistance coils of 10 ohms each were used in the two branches 
of the bridge, where 100 ohms resistances were generally employed.* As it was found 
impossible to allow the current to flow for even two or three seconds without unduly 
heating the wire, and as Villari’s “ shock-currents” would cause the resistance of the 
iron to appear greater than it should be when the B.C. was closed, and less than it 
should be when the B.C. was opened, the following plan was adopted :—The B.C. 
was, at intervals of 30 seconds, closed and immediately afterwards opened; and in this 
way it was found possible to obtain the value of the resistance-ratio of the iron and 
platinum so nearly, that on moving the sliding-piece to points on the iridio-platinum 
wire 20 millims. above or below the supposed balancing-point, a deflection of several 
divisions could be obtained in one direction or the other. As the contrary “ shock- 
currents” produced by closing and immediately opening the B.C. would not quite 
neutralise each other’s impulsive effects on the galvanometer-needle, it is obvious 
that the effects on the galvanometer, due simply to the fact of the sliding-piece 
being equal distances above or below the true balancing-point, would not produce 
equal deflections, but that, by taking the mean of these deflections, the true point 
might by easy calculation be determined. As a sample of the mode of experi¬ 
menting we will take the following case :—The true balancing-point seemed to be 
55 millims. to the right of the zero of the iridio-platinum wire, and 3 ohms were at 
this time in the circuit of the B.C. ; on moving the sliding-piece to 75 a deflection, 
on closing and immediately afterwards opening the B.C., of five divisions to the left 
was obtained (a left deflection would here indicate that the sliding-piece should be 
moved to the left in order to get the true balance). The sliding-piece was now moved. 
to 35, and a deflection of six divisions to the right was obtained on closing 
and opening the B.C. From this we learn that the true balancing-point would be 
35-t~ 6 *^ -=57 nearly. The resistance of 3 ohms was then removed, and similar 
observations gave 58 as the balancing-point. Immediately afterwards, the 3 ohms 
resistance having been again introduced, the point appeared to be 56. The true 
balancing-point when 3 ohms were in was therefore assumed to be and when 
* It may be perhaps as well to state here that when small resistances were being compared, 10 ohms 
instead of 100 ohms were generally employed, though this fact has not been always mentioned. 
