in reply to the Rev. Dr. Ritchie. 221 



the magnetism S is capable of inducing. 

 It is evident that a will represent the mag- 

 netism induced at the adjacent end of the 

 soft iron, and b that at the remote one: now 

 in this position of the soft iron B can have 

 no more magnetism than it receives from 

 S ; consequently, cannot exceed the power 

 of the magnet A, and therefore cannot in- 

 duce in it a higher degree of magnetism 

 than it possessed before the contact of B. 

 So far Dr. Ritchie's assertion holds good ; 

 but the circumstances are very different 

 under which the armature is placed in the 

 electro-magnet, or the common horse-shoe magnet referred to 

 in my paper. 



Now let B be placed, with its extremities in contact with 

 both poles of the magnet A ; then, according to the first case, 

 a will represent the magnetism at the S pole, and b that at the 

 N pole; but as S has induced in B a + i of magnetism, so N 

 must induce in B 6 + a of magnetism ; therefore 2 a will now 

 represent the magnetism at the S pole, and 2 b that at the N 

 pole. According to this reasoning, the contact of the armature 

 should tend to induce a + b oi' magnetism in the magnet A, 

 provided A has not received its maximum of magnetism, 

 or is not too hard to admit of further induction by the reaction 

 of the keeper. 



This is exactly what occurs, which will be manifest from 

 the following experiment. Take a common horse-shoe mag- 

 net, magnetized in the ordinary manner; then weaken this 

 magnet by gliding repeatedly the armature from the poles to 

 the neutral point: this mode of reducing the power of a mag- 

 net is much better than by applying its poles to the similar 

 poles of another magnet, as it does not alter the disposition of 

 the magnetism in the metal ; consequently the position of the 

 neutral point remains in the centre. The magnet being thus 

 treated, note accurately its magnetic power ; then apply a piece 

 of soft iron to its poles, in the manner in which the lifter is em- 

 ployed in the electro-magnet; and after the removal of the soft 

 iron, the magnet will be found to have acquired a considerable 

 increase of magnetic power. If the lifter be'slided from the neu- 

 tral point to tlie poles, its reaction will have been exerted more 

 generally upon the steel, and the magnetism induced by the 

 keeper will consequently be increased. By performing these 

 experiments upon magnets composed of steel of different de- 

 grees of hardness, it will be found that the softer the steel is 

 the greater will be the increase of the magnetism it acquires 



