1887.] On the Heating Effects of Electric Currents. £95 



The value of the constant " a " for the different metals is there 

 fore : — 





Inches. 



Centimetres. 





11684-0 . 



. . . 2886-0 





7948-4 . 



. . . 1964-0 





5258-0 . 



. . . 1299-0 





, 5203-7 . 



. . . 1285-0 





4860-7 . 



. . . 1201-0 





3190-9 . 



. . . 788-0 





, , 1800-6 . 



. . . 445-0 



Alloys (lead and tin, 2 to 1) 



. . 1455-5 . 



. . . 359-5 





1512-27. 



... 373-5 



The values in the second column are obtained from those in the first 



by multiplying the latter by ^.54)3 n = 



Since C = ad 2, I** gives the fusing current of any wire of a given 

 diameter d, inversely — 



will give the diameter of the wire which will fuse with a given 

 current C. Very useful tables can thus be calculated which would be 

 of service to the electric light engineer. 



[Jan. 5, 1888. — In all these experiments the results obtained on 

 wires finer than those recorded, viz., those below 10 mils, were ex- 

 cluded, because it was found that they did not follow the law of the 

 3/2 power. In the discussion which followed the reading of the paper, 

 Professor Ayrton pointed out that this must be so, and that it followed 

 from Mr. Box's researches of 1868* that the current required -to 

 maintain a fine wire of a given material at a given definite excess of 

 temperature is approximately directly proportional simply to the 

 thickness of the wire. This has been fully developed in a paper read 

 before the Society of Telegraph-Engineers and Electricians, November 

 24, lb87 ('Journal,' vol. 16, p. 539).] 



* 1 A Practical Treatise on Heat/ 1868. 



