SUPPOSED POLARITY OF DIAx\IAGNETIC BODIES. 
183 
together, will show the phenomena of revulsion very well ; and that according to the 
direction of the leaves, and not of the external form. Gold, silver, tin and copper 
have the revulsive effects thus greatly exalted. Antimony, as I have already shown, 
exhibits the effect well (2514. 2519.). Both it and bismuth can be made to give 
evidence of the induced currents produced in them w^hen they are used in thin plates, 
either single or associated, although, to avoid the influence of the diamagnetic force, 
a little attention is required to the moments of making and breaking contact between 
the voltaic battery and the electro-magnet. 
2687. Copper, when thus divided into plates, had its revulsive phenomena raised 
to a degree that I had not before observed. A piece of copper foil was annealed and 
tarnished by heat, and then folded up into a small square block, half an inch in the 
side and a quarter of an inch thick, containing seventy-two folds of the metal. This 
block was suspended by a silk film as before (2248.), and whilst at an angle of 30° or 
thereabouts with the equatorial line (2252.), the electro-magnet was excited ; it im- 
mediately advanced or turned until the angle was about 45° or 50°, and then stood 
still. Upon the interruption of the electric current at the magnet the revulsion came 
on very strongly, and the block turned back again, passed the equatorial line, and 
proceeded on until it formed an angle of 50° or 60° on the other side ; but instead of 
continuing to revolve in that direction as before (2315.), it then returned on its course, 
again passed the equatorial line, and almost reached the axial position before it stood 
still. In fact, as a mass, it vibrated to and fro about the equatorial line. 
2688. This however is a simple result of the principles of action formerly developed 
(2329. 2336.). The revulsion is due to the production of induced currents in the 
suspended mass during the falling of the magnetism of the electro-magnet ; and the 
effect of the action is to bring the axis of these induced currents parallel to the axis 
of force in the magnetic field. Consequently, if the time of the fall of magnetic force, 
and therefore of the currents dependent thereon, be greater than the time occupied 
by the revulsion of the copper block as far as the equatorial line, any further motion 
of it by momentum will be counteracted by a contrary force ; and if this force be 
strong enough the block will return. The conducting power of the copper and its 
division into laminse, tend to set up these currents very readily and with extra power ; 
and the very povver which they possess tends to make the time of a vibration so short, 
that two or even three vibrations can occur before the force of the electro-magnet has 
ceased to fall any further. The effect of time, both in the rising and falling of power, 
has been referred to on many former occasions (2170. 2650.), and is very beautifully 
seen here. 
2689. Returning to the subject of the assumed polarity of bismuth, I may and 
ought to refer to an experiment made by Reich, and described by Weber*, which, if 
* Taylor’s Scientific Memoirs, v. p. 480. 
