INTENSITY OF MAGNETIC AND DIAMAGNETIC FORCES. 715 



suspended bythree silk threads about 200 millim. in length from a 

 balance, which is sufficiently delicate to indicate a millegramme, 

 and which, excepting the axis of the beam, contains no iron. To 

 increase the action when the forces are weak, the glass is not 

 brought into contact with one only of the two poles of the great 

 electro-magnet, but the two keepers (C)* are applied to it, and 

 these are approximated by their rounded ends in such a manner 

 that their least distance apart amounts to 6 millim. ; and the 

 balance is so adjusted, that the watch-glass in the ring, when the 

 balance is counterpoised, simultaneously touches each half of 

 the keeper, and this at a single point. After the excitation of the 

 magnetism the watch-glass is attracted. In the scale suspended 

 at the other end of the beam, small leaden shot and then fine 

 sand or thin paper in small fragments are placed, until the 

 watch-glass is drawn away from the halves of the keeper. This 

 takes place with the greatest uniformity ; and when the forces 

 are small, after some pi'actice, the results of the different weigh- 

 ings do not differ from each other by more than 2 millegrammes. 

 The weight of the shot and sand or paper added is the measure 

 of the magnetic force in each case. 



6. As we are able to compare the intensity of the magnetism 

 of different magnetic substances, so we can also determine the 

 relative intensities of the diamagnetism of different diamagnetic 

 substances. For this purpose we require merely to measure the 

 repulsion which such substances experience from the pole of a 

 magnet ; and here we may again most conveniently make use of 

 the balance with the arrangement described in the previous 

 paragraph. With this view we may at once counterpoise the 

 substance to be tested, so that it comes into contact with, the 

 two portions of the keeper ; and after it has been repelled by 

 the excitation of magnetism in the electro-magnet, it may be 

 gradually loaded until it again comes into contact with the two 

 portions of the keeper ; or when the magnetism is excited, we 

 might adjust the balance as above ; and when the substance, in 

 consequence of the interruption of the cuiTcnt, ceases to be re- 

 pelled by the portions of the keeper and comes to rest upon 

 them, place weights in the pan until the substance again com- 

 mences to move from the two portions of the keeper. However, 

 I have decided in favour of another method, which j)ermits of 

 much more accurate determination. 



* Compare page 554. 



