68 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
traction which would suffice to double the width of the strip would be •'742 per unit. 
Also taking the ratio of lateral contraction to elongation to be *281, as was the case 
with iron (1), the total decrease of resistance due to mere change of form would 
be 1 ’562 ; and therefore on the whole there would seem to be an increase of specific 
resistance of ‘82, caused by stress sufficing to double the width of the strip. 
Experiment XVII. 
A strip of tin-foil of nearly the same dimensions as the iron-foil used in the last 
experiment was tested with a load of 8 kilogs., and a permanent decrease of resistance 
thereby produced = '001 per unit; a temporary decrease of ’002 per unit was caused 
by the application of the load, and it was calculated that the temporary decrease per 
unit effected by a stress of 1 grm. per square centimetre would amount to 2581X10“ 10 
as compared with 1055x10 “ n , the alteration caused by the same amount of longi¬ 
tudinal stress with tin wire. It would thus seem that, contrary to what takes place 
with the iron, the alteration produced by transverse traction is much greater (about 
23 times as great) than that caused by longitudinal traction; and if we suppose the 
elasticity of the foil to be even much less than that of the wire, there must be a very 
appreciable decrease of specific resistance; this latter, if the values of e and cr are the 
same for the foil and the wire, would be 70’0 per unit, i.e., the metal would have a 
specific resistance of y^th of its original specific resistance, if stress were applied 
transversely, sufficing to double the width of the foil. The corresponding alteration 
produced by the same amount of stress applied longitudinally to a wire of tin is, as we 
have seen, less than 2 per unit. 
Experiment XVIII. 
A strip of zinc-foil of nearly the same dimensions as the strips of iron and tin last 
used was tested with loads varying from 50 to 100 kilogs. A stress of 50 kilogs. 
produced a permanent decrease of resistance amounting to '0091 per unit, 70 kilogs. 
a decrease of ‘02 per unit, and with 100 kilogs. the foil was partly pulled away from 
the clamps ; before this, however, the temporary decrease of resistance produced by 
50 kilogs. was ascertained to be ‘028 per unit. After the accident the foil was 
securely reclamped, and now three trials with 50 kilogs. gave alteration of temporary 
resistance amounting to ‘010, - 017 and ’010 respectively, the load in the second of 
the three trials having been allowed to remain for a longer period, on the foil than was 
the case in the other two trials. It was then evident that the time during which the 
load was allowed to remain on the foil largely influenced the temporary alteration 
of resistance, and accordingly several experiments were made with a view to verify 
this fact, which was eventually abundantly established. In one of these experiments 
a load of 70 kilogs., after having been put on and taken off’several times, was allowed 
