524 



Mr. H. Tomlinson. The Influence of 



for the purpose, being held by the finger and thumb in the centre, and 

 rubbed with a resined glove. When rubbed, the wire yielded some- 

 times one note, and sometimes another about a semitone lower in 

 pitch. Both these notes were fairly clear, and from the higher of the 

 two a value for the modulus was deduced of 2276 X 10 6 grams per 

 square centimetre, whilst from the lower was obtained a value which 

 within the limits of error of observation was exactly equal to that 

 got by the static method ; so that we may regard 2089 X 10 6 grams 

 per square centimetre as representing with fair accuracy* the value of 

 " Young's modulus." 



There was no difficulty in finding the value of the simple rigidity in 

 the usual way, and this proved to be 724*8 X 10 6 grams per square 

 centimetre. From the above data the ratio of lateral contraction 

 to longitudinal extension was calculated to be 0*441. The values of 

 " Young's modulus " and of the simple rigidity are both high, and it 

 is very remarkable that the 10 per cent, of iridium added to the 

 platinum should have raised the former value more than 40 per 

 cent. 



Arrangement of the Platinum-iridium Wire for Observations on the 

 Alteration of Resistance produced by Longitudinal Traction. 



Experiment X. 



We will now turn to the experiments on the alteration of resistance 

 produced by longitudinal traction experiments, which it will be seen 

 resulted in a complete surprise in more ways than one. In the first 

 trials the wire was arranged in exactly the same manner as the steel 

 and magnesium wires had been, and the lever used for applying the 

 stress, but a German-silver wire was employed as the comparison - 

 wire. With only a permanent stress, due to the weight of the lever, 

 2 kilos, put on or taken off the lever caused an alteration of resist- 

 ance which required to be balanced by moving the sliding-piece 

 54*5 divisions, and when there was a permanent stress equivalent in 

 all to 3 kilos, on the end of the lever, an additional kilo, on or off 

 caused an alteration represented by 27*0 divisions, or almost exactly 

 half the number of divisions which were required with twice the 

 temporary load. Hence it was calculated that a stress of 1 gram per 

 square centimetre produced an increase of resistance per unit of 

 3049 X 10 -12 . The increase of resistance per unit divided by increase 

 of length per unit was 6*368, and the increase of specific resistance 

 per unit was 4*486. These results were very unexpected, inasmuch as 

 all the other alloys which have been examined, namely, brass, Grerman- 

 silver, and platinum-silver are altered by stress in their electrical 

 resistance far less than the pure metals of which they are composed, 



* This value would seem to be correct within at least 2 per cent. 



