1906-7.] Change in Lead Wires by Permanent Stretching. 19 
Tomlinson gives the following corresponding values : — 
Silver from *0094 to *0262 
Copper „ *063 to *013 
Iron (one specimen) „ *034 to *041 
Iron (another „ ) „ — -0183 to — ’0091 
Nickel 2*366 to +*509 
He did not study lead. 
Gray * determined the corresponding changes in the weight specific 
resistance, i.e. resistance of unit length of wire of unit weight, and gave 
the following results : — 
Copper *5 to *6 
Iron *7 to *8 
German Silver *50 to *55 
Compared with most of these results, the above determination would 
seem to show that the change, if any, in the specific resistance of lead, due 
to permanent stretching, is small as compared with the similar changes in 
copper, nickel, iron, etc. 
The volume of the lead wire was computed, from the readings given 
above, for the various stages. We obtained the following results: — 
Volume in Cubic Cms. 
3*312 
Length of Wire in Cms. 
196*2 
3*315 
204*4 
3*312 
3*306 
213*4 
221*4 
The precision measure of the volume, calculated as before, is *006. The 
values found above for the volume show no change that cannot be 
accounted for by the errors in the determination of section and length. 
The percentage increase of volume per unit percentage increase of 
length is, from the above results, not greater than ±‘012. 
Gray and Henderson *(• give the change in density of a lead wire from 
7*695 to 7*637, the wire being stretched until it broke, and the amount of 
elongation being 3*6 per cent. This corresponds to a *22 per cent, decrease 
in density per unit percentage increase of length. In our experiments the 
lead wire broke during the next stretch after it was of length 221*4 cms., 
the last length for which the results were obtained. Gray and Hender- 
* Trans. R S.E . , xxx. 369, 1880. t Proc. Roy. Soc., liv. 283, 1893. 
