216 Mr. 8. Tolver Preston on some Electromagnetic 
on its axis, when one terminal of a voltaic battery is main- 
tained in sliding contact with the equatorial part of the 
magnet, the other terminal in sliding contact with the pole 
of the magnet (a perfectly well-known experiment). 
Prof. Weber thinks that, because in this case the current 
flows during part of its course within the body of the magnet, 
the explanation of the rotation afforded by Ampere’s theory is 
“not at all” (keineswegs) applicable*. 
Prof. Weber says (translation):—“ The two effects (2. e. 
the rotation of a magnet on its axis, and the converse case of 
the rotation of a conductor about the magnet), which Ampere 
here endeavours to explain from like causes, are, however, in 
reality of different nature; and each demands its own expla- 
nation. The explanation given by him (Ampere) only applies 
to the rotation of the conductor about the magnet, discovered 
by Faraday, and not at all to the rotation of the magnet on 
its axis discovered by himself.” { 
But now I would call attention to the fact that the rotation 
of the magnet is found to take place apparently equally well 
when the current does not enter the body of the magnet at all, 
such as when the magnet is coated with a metallic sheath or 
cylinder, insulated from the body of the magnet by intervening 
cement or paper, so that the current flows along the sheath 
instead of entering the body of the magnet. 
If, then, the magnet rotates in this case (when no current 
enters its body), how, it may be asked, can it be said that 
Ampéere’s explanation of the rotation when the body of the 
magnet (instead of the metallic sheath) is used to conduct the 
current, is “ not at all”’ (keineswegs) applicable ? 
For Ampere’s theory, when the magnet is coated with the 
sheath, admittedly accounts for a force of rotation of the 
magnet (on its axis) equal to that with which (conversely) 
the external loop—cealled for brevity ‘‘ conductor ”-——would 
rotate about the magnet if it (the loop or “ conductor’) were 
free to move. Ampere’s theory therefore accounts for a 
force of rotation in the magnet equal to the opposite force of 
* Ampére, as is known, considered the rotation of the magnet on its 
axis to be due to the same cause as the (converse) rotation of the “con- 
ductor” (or external loop of wire) about the magnet, or he considered the 
effect to be a simple case of “ action and reaction” between the movable 
parts of the circuit ; ¢. e. whether the conductor revolved about the mag- 
net, or (conversely) the magnet rotated on its axis, depended simply on 
which was free to move. The movement (on Ampére’s view) depended 
on the reaction between the current in the conductor and the external 
magnetic field of the magnet, whereby the two tended to be twisted or to 
rotate in opposite directions. 
t+ Pogg. Ann. 1841, Bd. lit. S. 354. 
