504 



The Rev. T. R. Robinson's Experimental Researches on the 



Table XV. 



The great loss of magnetic force arising from the removal of either of the 

 connexions AB, CD (the former in particular) is remarkable. This case occurs 

 in 67, 70, and 77, and is equivalent to that of a bar magnet -whose length isc — k 

 lifting by one of its poles. Even in 77, where the effect is greatest, it is not 0-09 

 of the full po-wer of the magnet, and this proves that the excitation of the mole- 

 cules must be greatly diminished. It will hereafter be shown that in, for instance, 

 68, the induction through the keeper from B to A is nearly three-fourths of the 

 direct effect of the helix at A ; therefore, supposing that the resultant were even 

 the sum of its components, the cutting off this induction should still leave four- 

 sevenths of that resultant, instead of 003. It seems, therefore, that the brass 

 keeper reduces the excitation fully 29 times. In 78 the action is that of a mag- 

 net of half the length of 77, or y - k, and is proportionally less. The greater 

 loss caused by removing AB, than by CD, is easily explained from what is 

 stated in (c). In the first case, the induction from the second arm takes place 

 through the distance c — z — k,in the other, through the lesser one z+k. The 

 high ratio in 76 is also to be anticipated : in 68, 1^=549 is applied to two 

 inches of the arm at B, the effect of which is induced through z-\-k = T aX A; 

 in 76, one -^ = 400 acts through the same space, but in addition another helix 

 acts on the next 2 inches of the arm with equal energy, and at the inducing 



1 



