Effects of Mechanical Stress on Electrical Resistance. 285 



Original density before stretching = 8'8612 



Density after stretching almost to breaking = 8 '818 7 



This shows a decrease in density of fully ^ per cent., and is some- 

 what greater than what Mr. Tomlinson obtained for the copper which 

 he tested, his result being % per cent. In the present tests, the 

 stretching obtained was usually about 25 or 26 per cent, before 

 breaking. Diagram 1 is a stress-strain curve of the copper wire 

 used, the abscissae denoting percentage strain, the ordinates the stress 

 in kilogrammes per square centimetre of the original section of the 

 wire. This curve is given to show the nature of the wire. 



Lead Wire. A similar series of tests was made on a length of 

 lead wire, diameter O8 mm., the stretching being carried on till the 

 wire broke. This point was reached after a stretching of 3' 6 per 

 cent. The values for the original and final densities are as 

 follows : 



