Dr. Tyndall on Diamagnetism and Magnecrystallic Action, 187 



Preserving the current constant at 50°, the index of the tor- 

 sion-head was turned gently against the repulsion, and in this 

 way the ball was caused slowly to approach the magnet. The 

 repulsion continued until the glass fibre of the beam pointed to 

 2° ; here an attractive force suddenly manifested itself, the ball 

 passed speedily on to contact with the core end, to separate it 

 from which a torsion of 50° was requisite. 



The circuit was broken and the beam allowed to come to rest 

 at zero, a space of about yV^^ ^^ ^^ mch. intervening between 

 the ball and the end of the magnet ; on closing the circuit the 

 beam was attracted. The current was once more interrupted, 

 and the torsion-head so arranged, that the beam came to rest at 

 3° distant ; on establishing the current again the beam was re- 

 pelled. Between 0° and 3° there was a position of unstable 

 equilibrium for the beam; from this place to the end of the 

 magnet the attraction was triumphant, beyond this place repul- 

 sion prevailed. 



Here we see, that on approaching the pole, the attraction of 

 the magnetic particle mounts much more speedily than the re- 

 pulsion of the diamagnetic ball ; a result the reverse of that ar- 

 rived at by M. Pliicker, but most certainly coincident with that 

 which everybody who has studied electro-magnetic attractions 

 would expect. Shall we therefore conclude that ^ magnetism^ 

 increases more quickly than ^ diamagnetism V The experiment 

 by no means justifies so wide a generalization. If magnetism be 

 limited to the attraction of soft iron, then the above conclusion 

 would be correct ; but it is not so limited. Pliicker calls the 

 attraction of his watch-glass magnetism, the attraction of a salt 

 of iron bears the same name, and it so happens that the attrac- 

 tion of a salt of iron on approaching the poles increases incom- 

 parably more slowly than the attraction of iron itself. The proof 

 of this remarkable fact I will now proceed to furnish. 



From one end of a very fine balance a sphere of soft iron, ^th 

 of an inch in diameter, was suspended. Underneath, and about 

 ^th of an inch distant from the ball when the balance stood 

 horizontal, was the flat end of a straight electro-magnet. On 

 sending a current of 30° through the surrounding helix, the 

 ball was attracted, and the force necessary to efi*ect a separation 

 was measured : it amounted to 90 grammes. A plate of thin 

 window-glass was then placed upon the end of the magnet, and 

 the ball allowed to rest upon it. The weight necessary to eiFect 

 a separation, when the magnet was excited by the same current, 

 amounted to 1 gramme. Here an interval of about yV^t of an 

 inch was sufficient to reduce the attractive force to g^g*^ ^^ ^^^ 

 exerted in the case of contact. 



A sphere of sulphate of iron, of somewhat greater diameter than 



