Stress and Strain on the Properties of Matter. 515 



specific resistance of hard piano-steel wire is temporarily diminished 

 by very considerable longitudinal traction, I arranged a pair of hard 

 piano-steel wires in the same manner as the nickel wires, of which 

 already mention has been made, and proceeded to test one of them in 

 the usual manner, expecting that whilst moderate stress would 

 increase the specific resistance, excessive stress would diminish it. 

 The loading was accomplished by means of the lever before used, and 

 the results are given below : — 



Permanent 

 load on the 

 lever in kilos. 



Temporary 

 load in kilos. 



Increase of resistance 

 produced by loading 



in divisions of tlie 

 platino-iridium wire. 



Increase of 

 resistance per 

 kilo, on the 

 lever. 



1# 



4 



160 



40-00 



1 



6 



242 



40-33 



7 



6 



240 



40-00 



13 



6 



242 



40-33 



Mean 



40-17 



It will be seen that the increase of resistance produced by the load- 

 ing is proportional to the load throughout. The diameter of the wire 

 was 0*08246 cm., and the greatest total load on the wire was equal to 

 102 kilos., or 2 cwts. The greatest stress per square centimetre was 

 above 19,000 kilos., and only a little short of the breaking stress. 

 From the above results was deduced that the increase of resistance 

 per unit produced by a stress of 1 gram per square centimetre, was 

 1620 X 10 -12 , and this number so nearly agrees with those obtained for 

 the previously used specimens of piano-steel, that it was not con- 

 sidered necessary to determine the value of " Young's modulus " and 

 of the simple rigidity, in order to prove that, as in soft iron, the 

 specific resistance is increased, not only when moderate stresses are 

 employed but also for stresses close to the breaking load.~\ This last 

 experiment, however, confirms the fact mentioned in an earlier paper, J. 

 that the electrical resistance of steel is less increased than that of 

 iron, not only per gram stress on the square centimetre, but also per 

 unit increase of length. 



* This is the load due to the weight of the lever itself. 



f I afterwards found that Mr. Johnson must have miscalculated the amount of 

 temporary lengthening produced by the load used by him, as he gives it as 3 per 

 cent., though the wire was not loaded beyond the limits of elasticity. This last is- 

 an impossible result, as the greatest tensile strength of any piano-wire does not 

 admit of a temporary elongation of more than 16 per cent, without breaking. 

 " Proc. Eoy. Soc," vol. 26, p. 401. 



