534 Royal Society : — 



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

 the increase 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 increase per unit of resistance was 



for the steel wires 1875-5x10" , 



for the iron „ 2132-2 x 10" 12 , 



and for the brass „ 2244-9 x 10" 12 , 



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 5082 x 10 to 5665 x 10 , 



in the case of the iron wires from 4896 x 10" 13 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 

 force of 1 gramme on a centimetre of the material by the increase 

 of length produced by the stretching force, so as to obtain the in- 

 crease per unit of resistance when the wires are stretched 1 centi- 

 metre, a mean value of 3-525 was obtained for the steel wires, 

 3-951 for the iron wires, and 2-203 for the brass wires — thus 

 showing that, though the increase per unit of resistance for a 

 given stretching force is greater in brass than in iron or steel, 

 the increase per unit of resistance for a given lengthening of 

 the wire is much greater both in iron and steel than in brass. 



The torsional rigidity of the wires was next ascertained by the 

 method of vibrations, several trials being made with different 

 lengths of each wire; the results for different lengths of the 

 same wire agreed very closely indeed. 



From the values of torsional rigidity and the increase of length, 

 the diminution of section was calculated for a cubic centimetre of 

 each wire when stretched with a force of 1 gramme, assuming 

 the wire to be isotropic. Next the increase of resistance which 

 would result from mere lengthening of each wire and diminution 

 of section was determined, and it was ascertained that, on sub- 

 tracting this latter value from the total observed increase of 

 resistance, there was a considerable residue in the case of the 

 steel and iron wire, also a residue not so great in the brass. 

 This residual increase of resistance probably arises from increased 



