1886.] Electromagnets and the Magnetisation of Iron. 493 



of the superficial magnetism of the disk on the negative side.* In 

 calculating the force upon unit of area of one surface of the disk 

 itself, we must omit the half due to the surface in question. Thus 

 the force becomes 27rl 2 per unit of area of surface. To this must be 

 added the force due to the mutual action of the coil and the iron 



=ll- 



If, therefore, W=the grams weight supported per unit of area, we 

 have for the divided ring — 



and by giving to W and jg the values which have been found by 

 experiment to correspond with each other, we have the means of 

 finding corresponding values of J and |J. These are given in the 

 first and second columns of Table II, and are plotted as a curve in 



Table II. 



ft ' 



J- 



K. 



fX. 





3-9 



587 



151 -0 



1899-1 



7,390 



5-7 



735 



128-9 



1621 -3 



9,240 



10-3 



918 



89-1 



1121 -4 



11,550 



17 7 



1083 



61-2 



770-2 



13,630 



22-2 



1147 



51-7 



650-9 



14,450 



30-2 



1197 



39-7 



500-0 



15,100 



40 



1226 



30-7 



386-4 



15,460 



78 



L337 



17-1 



216-5 



16,880 



115 



1370 



11-9 



150-7 



17,330 



145 



1403 



9-7 



122-6 



17,770 



208 



1452 



7-0 



88-8 



18,470 



293 



1474 



5-0 



64-2 



18,820 



362 



14S9 



4-1 



52-7 



19,080 



427 



1504 



3-5 



45 -3 



19,330 



465 



1508 



3-2 



41 -8 



19,420 



503 



1510 



3 



38-7 



19,480 



557 



1517 



2-7 



35-2 



19,630 



585 



1530 



2-6 



33-9 



19,820 



fig. 1. Here again it will be seen that when p has exceeded the 

 value of about 200, the ratio g to f) no longer continues to diminish, 

 and the curve apparently becomes a straight line, the equation to 

 which is — 



|=0-19f +1418. 



I do not suggest that the portions of both the curves obtained for 

 W and p, and for | and p, which so far as the experiment goes 

 differ insensibly from straight lines, would, in fact, continue to 



* "Maxwell's " Electricity," vol. 2, §§ 396-9. 



