150 
each case. These two experiments gave results sometimes 
the same, at others differing 4 to 6 per cent in the annealed 
samples, 3 per cent in the hardened and tempered and 4 per 
cent in the bright wires, except in sample No. 6, where a 
difference of 20 per cent was observed. This probably 
accounts for the wide divergence of this sample. 
These variations may seem large, but it must be remem- 
bered that there is an essential difference between an obser- 
vation on electrical resistance and one on tensile strain. 
If we suppose the length L of wire under experiment 
divided into a great number of extremely short pieces 
etc., each one would have a slightly different breaking 
strain, hi, h^, etc., and electrical resistance, Ti, r^, etc., 
depending on the physical structure of each length li, 4, 
etc. Now the observed electrical resistance R of the wire L 
is the sum of all resistances of the parts li, h, etc., or 
Rr= S(r), while the observed breaking strain is not the mean, 
but the minimum breaking strain of all the pieces li, U, Ig, 
etc. It is an experimental fact, that the longer the piece of 
wire under experiment, the lower will be the breaking 
strain. 
Some of the results of the experiments are given in table 
B. Here we see again that the electrical resistance 
and breaking strain increase together in a pretty regular 
way, the variations observed probably being due partly to 
differences in temperature, but chiefly to the fact just 
explained, that in the electrical experiments we register a 
mean, while in the experiments on tensile strain we register a 
minimum observation. This law must be applied with due 
care and only where the samples have been drawn in the 
same way. 
