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



on the law of induction from molecule to molecule, which is altogether unknown. 

 Sir W. S. Harris has inferred, in the case of a permanent magnet inducting at 

 a distance, that this law is the inverse of the distance. Without, however, 

 inquiring how far his observations justify this conclusion, it manifestly cannot 

 apply in the present instance, as the facts rather indicate an exponential func- 

 tion. If the coercive force of the metal did not interfere, the negative logarithm 

 of the induction through iron should be proportional to the distance ; but the 

 law of this force also is unknown.* 



In examining the action of spires, and, still more, facts of induction, it is 

 necessary tohave a magnet of variable length, as no satisfactory conclusion can 

 be drawn if several be employed, owing to the various qualities of iron. It is 

 almost impossible to get two pieces of equal power, since the slightest diffe- 

 rence in forging, turning, or planing, influences this property. That which was 

 used in these experiments is of the same dimensions as the one described in 

 my former paper, differing in being soHd, and having its base of brass. The 

 cylinders are connected by a slide, composed of two pieces of the same iron, 

 each one and a quarter by two inches in section, in which semicircles are cut out 

 to receive them ; and by steady-pins and screws it can be firmly attached at any 

 height. From the excellence of the fitting the contact is very close ; and ex- 

 perience shows that it makes no interruption of the magnetic circuit. Setting 

 it to leave four inches of each cylinder, I found that with 0-85 current force, and 

 helices (F) containing 641 spires, the lift of the magnet is 817 lbs., when the 

 screws are tight, and 800 when they are slackened, and the contact maintained 

 by the attraction alone. If we allow for the decrease of magnetism mentioned 

 in my first paper, these two may be considered identical. The sufficiency of 

 the contact may also be inferred, from the parts of the cylinders below the slide 

 showino- no free magnetism. I found this to be the case even when this magnet 

 was excited to the highest power which I have yet obtained with it.f I may 

 add, that no part of the lifting power is due to the action of the spires on the 

 keeper or slide : when the helices, even excited to the great power mentioned 



* Bringing into contact with my magnet's N pole an iron tube, three-quarters of an inch in 

 diameter, and nine feet long, seven feet of it are N, and the remaining two S. 



t The cylinders were 2-1 long, the helices those already mentioned, and -^ = 3005-04, T = 81°-8, 

 />' was 1374'17. 



