Lifting Power of the Electro-Magnet. 



.521 



with the cylinders 2-1, and 0-954 with 101 (which case does not properly be- 

 long to this part of my subject, but is given for the comparison), instead of 

 0-849. 



This discrepancy is still more evident with the cylinder 8-1, where ((?), 

 (/f), and (/) are mere cylindric annuli, but where the ratio of (/) to (6^) is 

 found to be 0-8894 instead of 0-8217. 



The same is the case with respect to the influence of the cylinder's length ; 

 as is shown in the following Table of the ratio of the observed to the theoretic 

 /J, for helices equiradial to (5) ; in which I have also given it for the spirals 

 and three helices. 



Table XIV. 



Obs. u 



Cylindtr. r~ Helices. 



Calc. /i 



0-6 

 2-1 

 4-1 

 6-1 

 8-1 

 lO-l 



0-7031 

 0-9123 

 11904 

 1-4279 

 1-5294 

 1-6807 



Mean Diameter Q'"'- ^ Spiral, 

 of Spiral. Calc. /i 



C 1-520 

 D 1-700 

 £2-511 

 G 116 

 i/1-70 

 / 2-94 



0-7031 

 0-8303 

 0-9925 

 1-5221 

 l'43:i5 

 1-6438 



This divergence was not expected by me ; for the principles on which the 

 equation (1) is founded have been found to give correctly the action of helical 

 currents on each other, and their deflection of a magnetic needle. There is, 

 however, one marked difference between these and the case of the electro-mag- 

 net. The polarities of two currents cannot be in any way altered by their 

 mutual action ; those of the molecules of the needle are kept nearly permanent, 

 both in intensity and direction, by the coercive force of the hard steel ; so tliat 

 the ordinary methods of statics apply with certainty to them. In the case of 

 soft iron, both these vary -with the condition of the current, and according to 

 laws which I do not think are fully known. From the excess of activity of the 

 outer and lower spires, I am inclined to suspect that the resolution of the ex- 

 citing power in the direction of x and z is the main cause of error, though some 

 of it, as I have already indicated, must also belong to the mutual induction of 

 the molecules. 



