9G 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
breaking-load is nearly reached, when the contraction begins very suddenly to 
increase. 
Fig. 16 is from a photograph of a similar specimen, broken after stretching, and 
shows the gradual contraction from the ends to the centre and the sudden increase of 
contraction at this point. In measuring the diameter of the broken wires the gauge 
was placed close to the place of this sudden contraction. 
It appears also from Table XIII. that there is a certain amount of relationship 
between the breaking-load and the modulus of longitudinal elasticity; platinum, 
however, having too small a breaking-load and silver too high in comparison with the 
modulus of elasticity when contrasted with the other metals. 
It should be remembered that in the case of the former metal the loads at the 
critical points were less in proportion to the elasticity than was the case with the other 
metals, and if we regard the breaking-point as a third critical point, it would appear 
that these three critical points are in the case of well annealed wires related to each 
other roughly in the ratios of 1 : 1-^ : 2 or of 2 : 3 : 4. 
The Permanent Alteration of Specific Resistance caused by Stress. 
Traction. 
We have seen that the alteration of specific resistance which can be temporarily 
produced by longitudinal traction is very small, but then the temporary lengthening is 
small also. Now we can with wires which have been well annealed produce, in most 
cases, a far more considerable permanent increase of length, and it seemed desirable to 
ascertain whether there would be a correspondingly large change in the specific resis¬ 
tance of the substance. 
Three different modes of experimenting were tried :■—in the first, the wire to be 
tested and the comparison-wire were clamped into the blocks already described, and 
were then stretched at full length on the floor and side by side ; the short block, into 
which one end of the wire to be stretched was clamped, was placed behind two stout 
screws, which were screwed into the floor to about one half of their length, and about 
one inch apart; the wiie passed between these screws, and the other end of it which 
was clamped into the longer brass block was pulled by hand until it was cpiite straight' 
a mark on the block which traversed a wooden scale divided into millimetres, and 
secured to the floor by screws in a position parallel to the length ,of the wire, serving 
to measure the length of the wire. It was ascertained that after a little practice the 
readings could be depended upon to within at least 1 millim. or about - 04 V 0 the 
whole length. After thus measuring the length, the wires were placed in the air- 
chamber already described, and after the usual precautions, the ratio of their resis¬ 
tances tested ; a weight was now placed on the pulley for three minutes and then 
removed, when the wire was permitted to remain free from stress for 10 minutes, after 
which time a new determination of the ratio of the resistances of the wires was made. 
