156 
M$. H. TOMLINSON ON THE INFLUENCE OF STRESS 
Number of trial. 
Apparent alteration 
of resistance caused by 
magnetization. 
— signifies apparent decrease 
of resistance. 
1 
6-0 - 
2 
4-5 - 
3 
6-5 - 
4 
6-5 - 
5 
6-5 - 
6 
7-0 - 
7 
7-0 - 
8 
6-5 - 
9 
7-5 - 
Mean 
6-44- 
The B.C. was now taken off, and the direct action of the coil on the galvanometer 
was found to he 6'50. There would therefore, on the whole, appear to be an increase 
of resistance of O'G. The B.C. was again put on, and a similar set of observations to 
the above produced when the the M.C. was reversed, an apparent increase of 7'80 
divisions ; whilst the direct action of the coil on the galvanometer caused a deflection 
of 8'00 in the same direction, so that now, on the whole, there would appear to be a 
decrease of resistance represented by '20 division of the scale. The mean result 
of the two sets of observations would give a decrease of resistance represented 
by '07 division of the scale. Now, in this case, the galvanometer had been made 
so sensitive by proper use of the adjusting magnet that an alteration of 'I ohm on 
one of the two sides of the bridge, containing each 100 ohms, caused a deflection 
of 300 divisions of the scale; accordingly 1 division of the scale would represent 
an alteration of resistance amounting to 300 V 00 P er unit, and '07 an alteration 
of less than one in four millions. It is needless to say that this experiment shows 
that there is no reliable change of resistance to be detected even with the com¬ 
paratively large magnetizing force employed in this case—a force which would be 
more than 480 times that of the earth’s magnetic horizontal force at tire place. The 
copper in this case was the ordinary copper wire usually employed for electrical 
purposes, but other experiments were made with chemically pure copper, and these 
all yielded results quite as negative as those just recorded. There is no doubt that 
the electrical resistance of copper is altered by magnetization; but in order to detect 
such alteration we should, in all probability, require the aid of a very powerful 
electromagnet* and a galvanometer in a very sensitive condition. Such change 
of resistance must be exceedingly small even with the most powerful magnetizing 
force that we can at the present time bring to bear—much smaller than would seem to 
* One miglit, perhaps, by adopting the same plan as that used with the zinc foil, succeed in obtaining 
evidence of alteration of resistance in the case of copper foil. 
