1891.] on the Molecular Process in Magnetic Induction. 



391 



magDcts are got by taking to pieces numbers of little pocket compasses. 

 The pivots are cemented to a glass plate, through which the light 

 })asses in such a way as to project the shadows of the magnets on the 

 screen. The magnetic force is apj)lied by means of two coils, one on 

 either side of the assemblage of magnets and out of the way of the 

 light, which together produce a nearly uniform magnetic field through- 

 out the whole group. You see this when I make manifest the field 

 in a well-known fashion, by dropping iron filings on the plate. 



We shall first put a single pivoted magnet on the plate. So long 

 as no field acts it is free to point anyhow — there is no direction it 

 prefers to any other. As soon as I apply even a very weak field it 

 responds, turning at once into the exact direction of the applied force, 

 for there was nothing (beyond a trifling friction at the pivot) to 

 prevent it from turning. 



Now try two magnets. I have cut off the current, so that there is 

 at present no field, but you see at once that the pair has, so to speak, 

 a will of its own. I may shake or disturb them as I please, but they 

 insist on taking up a j)osition (Fig. 3) with the north end of one as 

 close as possible to the south end of the other. If disturbed they 

 return to it : this configuration is highly stable. Watch what happens 

 when the magnetic field acts with gradually growing strength. At 

 first, so long as the field is weak (Fig. 4), there is but little deflection ; 



Fm. P. 



Fig 4. 



but as the deflection increases it is evident that the stability is being 

 lost, the state is getting more and more critical, until (Fig. 5) the tie 

 that holds them together seems to break, and they suddenly turn, with 

 violent swinging, into almost perfect alignment with the magnetic 

 force H. Kow 1 gradually remove the force, and you see that they 

 are slow to return, but a stage comes when they swing back, and a 

 comj)lete removal of the force brings them into the condition with 

 which we began (Fig. 3). 



If we were to picture a piece of iron as formed of a vast number of 

 such i^airs of molecular magnets, each jiair far enough from its ueigh- 



O T-, i) 



