Lifting Power of the Electro-Magnet. 527 



base opposing the action of the current on the cylinders. In No. 212 the 

 upper half of what in ordinary cases is the N cylinder was S, the lower N, and 

 vice versa in the other cylinder; the polarities of the keeper remaining un- 

 changed, but the natural distribution was restored when the current ceased. 



The values of A are greater than those corresponding to the same L in the 

 iron magnet, and their maximum would probably be higher, but their increase 

 with -^ shows that it was not reached, though No. 202 would probably have 

 given it. 



A is, of course, still greater than in the compound magnet, bearing a con- 

 siderable proportion to the whole force. In the two the lengths of saturated 

 steel are as 7 : 4; the \s as 7 : 5-3, the excess in the latter being due to the 

 iron keeper. I have already remarked that when the steel has been fully mag- 

 netized in one direction, that tendency continues, so that even after several re- 

 versions of its polarity, it is far more easily magnetized in the original direction 

 than the other. Analogous to this is the fact, which is shown still more clearly 

 than in Table xxxn., that a positive >^ gives a larger L than a negative; and 

 that the latter, unless it be great, merely destroys a portion of \, the L being 

 scarcely diiferent from what is left of that. Thus in Nos. 208, 212, the posi- 

 tive ^ gives 32'77 = \ -f 17'32 ; therefore, on any supposition of the resolution 

 of magnetic force, we should expect its effect, when negative, to overpower \, 

 and give a negative result; we, however, find Z = -f 6'92 and \ = 9'04, with 

 the additional fact that under such circumstances the state A has no existence. 

 In 207, 213, + 390 gives 38-7, and 14-4 ; - 390 gives -f 3-4, and 6-0 : the next 

 pair give, the positive 44-9, and 13-6 ; the negative — 2*9, and — 1-3. It seems 

 from this, that steel (and, though in a less degree, iron) undergoes by being 

 fully magnetized some change, which continues when the magnetism is de- 

 stroyed. In virtue of that change it resists an opposite excitation, which may 

 lessen its existing polarity, or even produce an opposite one ; but the bias re- 

 mains until an overwhelming power is applied which is sufficient to destroy it 

 altogether, and substitute an opposite one in its stead. This, however, must 

 far surpass that which originally produced it. In the present instance even 

 the high -f 1045 falls far short of this; and I think it not improbable that, had 

 the magnet been left to itself, these \s, which were diminished by reversed ex- 

 citation, would have returned to their original magnitudes. 



3z2 



