to Magneo-ystallic Action. 



129 



For the sake of disencumbering the mind of all considerations 

 save those which belong to pure magnecrystallic action, we will 

 suppose, as before, the bodies experimented with to be spherical. 



Let the dot at a-, fig. 3, be Fig. 3. 



the intersection of the silken 

 axes with Mr. Faraday's strip 

 of card ; and on the end of the 

 strip, let the sphere of sulphate 

 of iron be placed with its mag- 

 necrystallic axis ab at right 

 angles to the length of the strip. 

 This line, as I have already 

 shown*, is that of most in- 

 tense magnetization through the 

 crystals. The forces acting on the sphere in its present position 

 are exactly similar to those acting upon the carbonate of iron in 

 fig. 2. A residual " couple " will a]iply itself at the extremities 

 of ab, as indicated by the arrows, and would, if the sphere were 

 free to turn round its centre of gravity, set the line ab parallel 

 to the magnetic axis. But the sphere is here rigidly connected 

 with a lever moveable round its own axis of suspension, and it is 

 easy to state the mechanical result that must follow from this 

 arrangement. To obtain the moments of the two forces acting 

 upon a and b, we have to multiply each of them by its distance 

 from the axis x. In front of a fiat pole such as that made use 

 of by Mr. Faraday in these experiments, the force diminishes 

 very slowly as we recede from the pole, so that the attraction of 

 a does not so far exceed the repulsion of b as to prevent the pro- 

 duct of the latter into xz from exceeding that of the former into 

 xy, and consequently the paramagnetic sphere must recede from 

 the pole. 



In his next experiment, Mr. Fig. 4. 



Faraday removed the pole S and 

 allowed the pole N to act upon 

 the crystal as in fig. 4. In this 

 case it will be seen that the end 

 nearest the pole, and therefore the 

 most strongly attracted, is also 

 at the greatest distance from the 

 axis of rotation. Hence the 



N 



sphere must approach the pole, 

 as it does in the experiment. 



When the strip of card is revolved 90*^, we have the state of 

 things shown in fig. 5; and when it is revolved 180°, we have 



* Phil. Maji. S. A. vol. ii. p. \19: 



