1890.] Molecular Theory of Induced Magnetism. 345 



Fio. 1. 



is wanted, and the board slips into a large frame or open sided flat 

 box wound round the top, bottom, and two sides with a coil through 

 which an adjustable current may be passed to expose the group to a 

 nearly homogeneous external magnetic force. The coil is wound in 

 a single very open layer, through which a sufficiently good view of the 

 group inside is obtained.* A liquid rheostat with a sliding terminal 

 is used to secure continuity in varying the magnetic force. It is 

 scarcely necessary to add that the magnetic force which is applied to 

 the group is too weak to have any material effect on the magnetism 

 of individual bars. It alters their alignment only, just as a magnetic 

 force alters the alignment of Weber's molecular magnets. 



When a number of these magnets are grouped either in a regular 

 pattern or at random, and are left after disturbance to come to rest 

 free from external magnetic force, they of course assume a form 

 which has no resultant magnetic moment, provided the number be 

 sufficiently great but it is apparent that they do not arrange 

 themselves in closed chains. Any such configuration would in 

 general be unstable. Many stable configurations admit of being 

 formed, and if the magnets are again disturbed and left to settle 

 the chances are much against any one configuration immediately 

 repeating itself. One general characteristic of these configurations 

 is that they contain lines consisting of two, three, or more magnets, 

 each member of a line being strongly controlled by its next 

 neighbours in that line, and but little influenced by neighbours 

 which lie off the line on either side. Thus, if there are two magnets 

 simply, they form (as might be anticipated) a highly stable pair, 

 thus : 



* In showing the experiments, the board with the magnets on it may, of course, 

 be placed in clear view above the coil ; the U is then made larger, or a stronger 

 current is used. 



VOL. XLVIII. 2 B 



