Temperature of Wires under Pulling Stress. 97 



to the table, rests in the V. A third foot, rounded, rests on 

 the other platform, which is plane and polished. On the top 

 of this little table, which is a square of about 1 \ centimetre 

 in the side, there is fastened a perfectly plane parallel Stein- 

 heil mirror ; and a telescope with cross wires, looking down 

 very nearly vertically on the mirror, views, reflected in the 

 mirror, a half-millimetre scale suitably placed. 



It will be seen at once that if the two wires were to elon- 

 gate equally with rise of temperature, their extremities would 

 go down together and almost the only effect (not absolutely 

 of course) on the scale-reading would be to alter somewhat 

 the focus. But if one wire elongates more than the other the 

 mirror is tilted, and the change in the scale-reading readily 

 gives the amount of relative displacement of the ends of 

 the wire. 



The arrangement works in the most satisfactory way, and 

 it now only remains for me to state the results. I must 

 remark, however, that it was exceedingly difficult to make an 

 exact estimate of the temperature of the tube, even when the 

 steam was running strongly through it. Thermometers in- 

 serted by means of corks in holes provided for the purpose 

 showed that differences of 2° or 3° (I think not so much as 

 5°, however) existed at different parts of the tube. 



This being understood, I may say that the range of tem- 

 perature in the various experiments was from 15° 0. or 16° C. 

 (cold) to 98° or 99° (hot), or about 83° C. The length of 

 copper wire experimented on was 530 centimetres. The 

 difference of expansions observed was 0*14 millim. or 0*014 

 centimetre, the heavily loaded wire going down most. This 

 gives a relative expansion of 26 x 10 -6 per centimetre for a 

 change of temperature of 83° ; or 0*314 x 10~ 6 per centimetre 

 per degree. 



I find the linear expansion of copper per degree stated at 

 about 17*2 x 10 ~ 6 , and thus the ratio of this extra expansion to 

 the total expansion is 3*14/172, or about ^. 



With regard to platinoid wire — after more than three 

 months of daily heating and cooling, the wires (0*35 millim. 

 in diameter) came to a thoroughly permanent condition. On a 

 length of 490 centimetres a relative extra extension of 0*111 

 millim. or 0*011 centim. was observed for a change of 

 temperature of 83°, and as with copper the heavily loaded 

 wire experienced most elongation. These numbers give 

 22*4 x 10 -6 as the extra expansion per centimetre, or 

 0*27 x 10~ 6 per centimetre per degree Centigrade. 



The linear expansion of platinoid was unknown, though it 

 might be supposed to be something not very different from that 



