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



on each molecule, are the coercive force C; and the differences of polar attrac- 

 tions across the molecule itself M, and across the intervals between it and those 

 which adjoin it D. At the contact of the keeper this interval must be far 

 greater than in the continuous iron ; and the constant of attraction may also be 

 different, but I think the attractions will be in a constant, perhaps an assign- 

 able, ratio.* If in this state of things the current in the helices be stopped, the 

 polarities of the molecules tend to re-unite by the forces M—D, and are pre- 

 vented by the force C, which now maintains the magnetic state as it opposed 

 its production. As M must be always greater than Z>, and, as I have said, pro- 

 portional to it, the magnetism must sink until M— D= C, and then remain 

 permanent. It has long been known that the keeper of an electro-magnet ad- 

 heres to it with considerable force when the current ceases ; but I am not 

 aware that the meaning of this fact has been interpreted, or measm-es of it taken. 

 Now lifting the keeper, D is destroyed at the polar surfaces, and the forces are 

 M— C, so that the magnetism will decrease till J/= C, but will not necessarily 

 vanish even in this case. 



It is not my intention to go fiurther into the theory of electro-magnetism, 

 which I hope will be fully developed by the able geometrician to whom I have 

 ah'eady referred ; and I merely call attention to these elementary principles of 

 it for the purpose of indicating the sort of information which I have endea- 

 voured to obtain, and the way in which it seems to bear on these molecular 

 forces. 



The power of an electro-magnet may be examined either by measuring the 

 force required to detach a keeper from its poles ; secondly, by observing its 

 attraction of a mass of iron at a small distance ; or thirdly, by its deflection of 

 a magnetic needle. The second of these methods appears to me objectionable, 

 from the complication introduced by the rapid curvature of the lines of mag- 

 netic action near the poles, and from the great diminution of the forces by a very 

 small interval ; this is even more felt in the third, as the needle must be placed 

 at a very considerable distance from the magnet. In both the varying distri- 

 bution of the magnetism must be taken into account, and neither of them seems 



* The contact will be closer when the attraction is powerful, and therefore the adhesion of 

 the keeper something greater than what is due to the mere intensity of the magnetism, but I do 

 not know whether this effect is appreciable. 



