418 ON EFFECTS OF STRAIN ON ELECTRIC CONDUCTIVITY. 
homogeneous isotropic conductor, strain it (without altering its shape) so as to 
diminish its volume in the ratio Tee then its conductivity, being / in the 
unstrained state, becomes =4(1+ pé). 
An approximate evaluation of this constant is now possible, experiments on 
increase of resistance for stretched brass wires being made by Mr Tomuinson 
(Proc. Roy. Soc., 1877). According to his results a brass cube-centimeter 
opposing a resistance =719°4 x 10~* (mean) to the passage of a current between 
its faces, being longitudinally strained to an extent \=10120 x 10-” exhibits 
an increase of resistance (in the direction of strain) equal to 1704 x 10-”. 
Mr Tomutnson adopts for brass the value, 
 §= 2-208. 
Now, taking for brass »=0°34 (rather uncertain), and substituting for o 
the number given above, we calculate 
p=0°75, 
