Lifting Power of the Electro -Magnet. 



505 



distance 9", and so on through the five. A similar addition is made to the in- 

 ducing forces of the arm AC, so that tlie effect may easily be four times what 

 it is in the other case. I may remark tliat the removal of the slide, especially 

 when the arms are of considerable length, gives an approximation to the case of 

 c infinite, and that though an increase of the number of spires and the quantity 

 of current will augment the effect greatly, yet it is not probable that any finite 

 amount of them M'ill equal tlie influence of closing the circuit. 



That the force L is composed as stated in (c), may be shown by measuring 

 it in different positions of the slide CD, while the helices are kept in a given 

 position. In this case the components (z), (z + k), with a given i^, should be 

 invariable, and any differences which are found can only depend on the other 

 two. In general, two positions were chosen for each value of c, one in contact 

 with the keeper, the other with the slide. In this last case the forces excited 

 at C and D are the same which in the other are measured at A and B, and as in 

 both c is the same, the change depends on z only. I therefore hoped that by 

 comparing the series Z^ I might ascertain the effect of the circuit, and, if succes- 

 ful in that, comparing each L, with its own L^, I might get the law oi(z). The 

 first sets were made with the spirals A, with ^jr nearly 124-69, and reduced ex- 

 actly to it by interpolation from Table viii. 



Table XVI. 



