Lifting Power of the Electro-Magnet. 



507 



miuimum effect ; but as the influence of the former is not very great, it seems to 

 follow nearly the same march. The fourth column gives, for each pair, the 

 ratio of the effect with the spirals at keeper, to that with them at slide. This 

 ratio decreases much more slowly than either of the preceding ; for instance, a 

 change of c from 16-2 to 32'2 changes the ratio of Z,, from 0-93 to 0'51 ; but 

 the same change of c, combined with that of z from 0"07 to 10*07, only alters 

 this from 0-95 to 0-88. The comparatively small influence of z is still more 

 plainly shown by the sixth column, which gives for c = 32-2 the ratios of the 

 maximum to the L obtained with the z in the fifth column by Nos. 88-93. 

 A similar series made with the helices F.F\ and with •\^ = 549-08, is 



Table XVIII. 



The ratios, given as before, are as follows :- 



Table XIX. 



