including an Ewamination of the Magnetic Field. 339 



Suppose this line to be cde. Commencing at A, the intensity of 

 attraction towards this face decreases as we approach the centre dj 

 and at this point it is equihbrated by the equal and opposite 

 attraction towards B. Beyond c? the attraction towards A be- 

 comes negative^ that is, it is now in the direction c?B. The point 

 d will be a position of stable equilibrium for a diamagnetic sphere, 

 and of unstable equilibrium for a magnetic sphere. But if, 

 through the introduction of some extraneous agency, the line of 

 distribution be shifted, say to c'c^V, the point d will be no longer 

 a position of equilibrium ; the diamagnetic sphere will move from 

 this point to d', and the magnetic sphere will move to the pole A. 



17. For the purpose of investigating whether any change of 

 this nature takes place in the magnetic field when an electric 

 current passes through it, I attached a small sphere of carbonate 

 of iron to the end of a slender beam of light wood ; and balan- 

 cing it by a little copper weight fixed to the other end, the beam 

 was suspended horizontally from a silk fibre. Attaching the 

 fibre to a moveable point of suspension, the little sphere could 

 be caused to dip into the interior of the helix as it stood between 

 the poles, and to traverse the magnetic field as a kind of feeler. 

 The law of its action being that it passes from weaker to stronger 

 places of force, we have in it a ready and simple means of testing 

 the relative force of various points of action. The point of the 

 beam to which the fibre was attached being cut by the axis of 

 the helix produced, and the sphere being also on the same level 

 with the axis, when the magnet was excited* it passed into the 

 position occupied by the hard line in fig. 2, thus resting against 

 the interior of the helix a little within 

 its edge. On sending a current 

 through the helix, which in the upper 

 half thereof had the direction of the 

 arrow, the sphere loosed from its 

 position, sailed gently across the field, 

 and came to rest in the position of 

 the dotted line. If, while thus sail- 

 ing, the direction of the current in 

 the helix, or of the current by which the magnet was excited, 

 became reversed, the sphere was arrested in its course and brought 

 back to its original position. In like manner, when the position 

 of the sphere between the poles was that of the dotted line, a 

 current sent through the helix in a direction opposed to the 

 arrow, caused the sphere to pass over into the position of the 

 hard line. 



18. The sphere was next introduced within the opposite edge 



* One of Bunsen's cells was found sufficient; when the magnetic power 

 was high, the change caused by the current was not sufficient to deflect 

 the beam. 



