Lifting Power of the Electro-Magnet. 



305 



Hence 



Then I obtained 



I' -I 

 l(T'-T) 



+ 



1(1 





X (r-60^) + &c. 





'=461-55 .... !r=167°0 .... F= 1-5830 

 = 480-94 69-1 1-5868 



= 583-04 174-3 2-6198 



= 601-83 70-5 2-6198 



:'= 646-80 175.4 35634 



= 661-62 73-8 3-5761 



'=329-39 172-5 08613 



= 343-08 69-3 0-8608 



Interpolating for the difference of F in each pair,* I obtain from these- 



Mean, . - 0000322 



The three first might induce a suspicion that t diminishes as F increases ; 

 but the fourth disproves this ; and as the third set was less consistent than the 

 rest, I regard the difference as mere error. I use the value 0-00033. 



Subsequent to these experiments Dr. Lloyd has discovered that the induc- 

 tive power of terrestrial magnetism is increased by a small elevation of tempera- 

 ture. Before this came to my knowledge I had applied the wire of these 

 helices to other purposes, or I would have examined the coefficient t at inter- 

 mediate temperatures ; I have, however, made a similar observation with 

 respect to steel electro-magnets, and suspect it depends on the coercive force 

 bearing a high ratio to the inducing force. In the present instances I do not 



* The interpolation was deduced from a special series ; these values of i not being comparable 

 to those of the Table, as the helices have only half the number of spires, are of less diameter, and 

 their tin bobbins add something to the mass of the magnet. 



