354 PLiJCKER ox THE REPULSION OF THE OPTIC AXES 



magnetism and diamagnetism, as also the rotation of the planes 

 of polarization of light by magnetic action, I had a powerful 

 electro-magnet constructed by Etter, the university mechani- 

 cian, under my own superintendence; and to be sure of obtain- 

 ing at least the same action, the iron nucleus in it was made of 

 the same dimensions as those specified by Faraday for his large 

 horse-shoe magnets, substituting the Parisian for the English 

 foot. The surfaces of the ends of the poles were therefore circles, 

 the diameter of which was 102 millim. (3| French inches), and 

 the centres of which were 284 millim. (9| French inches) apart. 

 The iron nucleus weighs 84 kilogrammes, and each of its two 

 perpendicular arms is covered with four layers of copper wire, 

 each of which consists of ninety-two coils. This wire is 4*36 

 millim. in thickness (2 Rhenish lines), whilst the wire of Fara- 

 day's magnet was O"!/ of an English inch. On theoretical 

 grounds I chose the greatest thickness which could be conve- 

 niently obtained in wire which had been well heated to redness 

 and covered. The wire weighs about 35 kilogrammes. The 

 coil of wire extends to the surfaces of the poles ; to fit each of 

 these, an appendage of soft iron is ground ; the surface of this is 

 of the same diameter, and 48 millim. in height. The two appen- 

 dages are perforated in the centre of their height, and in the 

 perforations, which are 20 millim. in diameter, two moveable 

 cylinders of soft iron, which fit and are conically pointed at their 

 extremities, are inserted and fixed by screws. The conical 

 apices, in which the magnetic action is concentrated, can be ap- 

 proximated or separated from each other at will, and are remo- 

 vable either in connexion with, or separately from the appendages. 

 A glass case, containing a Coulomb's torsion-balance, is placed 

 upon the leaf of a table, which can be raised or lowered, and is 

 furnished with two round holes, through which the arms of the 

 electro-magnet pass. A strong thread, composed of a large 

 number of separate silkworm threads, winds up and down upon 

 the arm of the balance, to which heavy bodies, weighing as much 

 as half a kilogramme and more, can be immediately suspended, 

 for instance in a light little boat, to a small hook. As regards 

 the suspension of light bodies, from the first, in my magnetic 

 and diamagnetic experiments, I found it requisite to dispense 

 with this boat, since I could not obtain any substance which, 

 when placed between the two poles of my electro-magnet, did 

 not appear either magnetic or diamagnetic. I suspend all such 



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