452 Mr. H. Tomlinson on the Increase in Resistance [Dec. 21, 



4 pianoforte steel wires, 1 wire of commercial steel, 3 iron wires, and 

 4 brass wires were examined with several different weights. The wires 

 taken were of various sections, and it was found that in each case the 

 increase of resistance was " exactly proportional to the stretching force," 

 the stretching not being carried beyond the limit of elasticity of each 

 wire. The resistance of a cubic centimetre of each wire was then deter- 

 mined, also the increase of resistance which a cubic centimetre of each 

 wire would experience when stretched by a force of 1 gramme in the 

 same direction as the passage of the current was calculated from the 

 observations made. The former values varied from 



1574-8 X 10"^ to 1882-4 x 10"^ in the case of steel, from 

 . 1200-8 X 10"^ to 1291-0 X 10^^ in the case of iron, and from 

 656-7 X 10"^ to 782-2 x 10"^ in the case of brass ; 

 the latter values varied from 



2982 X 10"'^ to 3511 x 10"^^ in the case of steel, from 



2557 X 10~^^ to 2712 x 10"^^ in the case of iron, and from 



1565 X 10"^^ to 1843 x 10"^^ in the case of brass, 



the numbers in each case representing so many ohms. 



On dividing the latter values by the former, it was found that the in- 

 crease per unit of resistance for a stretching force of 1 gramme on a 

 cubic centimetre of each wire was nearly the same for wires of the same 

 material, but differed with wires of different materials. The mean in- 

 crease per unit of resistance was 



-12 



for the steel wires 1875*5 X 10 

 for the iron „ 2132-2 x 10" , 



12 

 .-12 



and for the brass „ 2244-9 x 10" , 



the greatest departure from the mean value being 



for the steel less than 2-7 per cent., 

 for the iron about 3-0 per cent., 

 and for the brass about 8-5 per cent. 



The temporary increase of length which a cubic centimetre of each 

 wire would experience on being stretched with a force of 1 gramme was 

 then calculated from observations which had been made in the usual 

 manner with the cathetometer ; this increase of length was found to vary 



in the case of 3 steel wires from 5082x10 to 5665 X 10 , 

 in the case of the iron wires from 4896 x 10"^^ to 5938 X 10~^^ 

 and in the case of 1 brass wire was 10120 X 10"^^. 



On dividing the increase per unit of resistance for a stretching forc©^ 



