166 Dr. Tyndall on Diamagnetism and Magnecrystallic Action* 



poles so near each other that the bark had barely room to swing 

 between them, it set itself, like a diamagnetic substance, with its 

 length perpendicular to the line which united the two poles. On 

 removing the poles to a distance, or on raising the bark to a 

 certain height above them, it turned round and set its length 

 parallel to the line joining the poles. As is usual, we shall call 

 the former position the equatorial, and the latter position the 

 aonal. Thus when the poles were near, diamagnetism was pre- 

 dominant, and caused the mass to set equatorial ; when the poles 

 were distant, magnetism, according to the notion of M. Pliicker, 

 was predominant, and caused the mass to set axial. From this 

 he concludes, " That in the cherry-tree bark two distinct forces 

 are perpetually active ; and that one of them, the magnetic, decreases 

 more slowly with the distance than the other, the diamagnetic. 



In a later memoir* this predominance of the diamagnetic force 

 at a short distance is affirmed by M. Pliicker to be due to the 

 more general law, that when a magnet operates upon a substance 

 made up of magnetic and diamagnetic constituents, if the power 

 of the magnet be increased, the diamagnetism of the substance 

 increases in a much quicker ratio than the magnetism ; so that 

 without altering the distance between it and the magnet, the 

 same substance might at one time be attracted and at another 

 time repelled by merely vaiying the strength of the exciting 

 current. 



This assertion is supported by a number of experiments, in 

 which a watch-glass containing mercury was suspended from one 

 end of a balance. The watch-glass was magnetic, the mercury 

 was diamagnetic. When the glass was suspended at a height of 

 3-5 millims. above the pole of the magnet, and the latter was 

 excited by a battery of four cells, an attraction of one mille- 

 gramme was obsei*ved ; when the magnet was excited by eight 

 cells, the attraction passed over into a repulsion of the same 

 amount. 



It is to be regretted that M. Pliicker, instead of giving us the 

 actual strength of the exciting cun'ent, has thought proper to 

 mention merely the number of cells employed. From this we 

 can get no definite notion as to the amount of magnetic force 

 evolved in the respective cases. It depends of course upon the 

 nature of the circuit whether the current increases with the 

 number of cells or not. If the exterior resistance be small, an 

 advance from four to eight cells will make very little difference ; 

 if the said resistance be a vanishing quantity, one cell is as good 

 as a million t. 



* Poggendorff' 8 Annalen, vol. Ixxv. p. 413. 



t The usual arrangement of the cells is here assumed ; that is, where 

 the negative component of one cell is connected with the positive compo- 

 nent of the next. 



