bar magnets two feet long, one inch and a half wide, and half an inch
thick; and, by rendering the galvanometer (87.) a little more delicate,
with the most striking results. Ferro-electro-magnets, as those of Moll,
Henry, &c. (57.), are very powerful. It is very essential, when making
experiments on different substances, that thermo-electric effects (produced
by contact of the fingers, &c.) be avoided, or at least appreciated and
accounted for; they are easily distinguished by their permanency, and their
independence of the magnets, or of the direction of the motion.

114. The relation which holds between the magnetic pole, the moving wire or
metal, and the direction of the current evolved, i.e. _the law_ which
governs the evolution of electricity by magneto-electric induction, is very
simple, although rather difficult to express. If in fig. 24, PN represent a
horizontal wire passing by a marked magnetic pole, so that the direction of
its motion shall coincide with the curved line proceeding from below
upwards; or if its motion parallel to itself be in a line tangential to the
curved line, but in the general direction of the arrows; or if it pass the
pole in other directions, but so as to cut the magnetic curves[A] in the
same general direction, or on the same side as they would be cut by the
wire if moving along the dotted curved line;--then the current of
electricity in the wire is from P to N. If it be carried in the reverse
directions, the electric current will be from N to P. Or if the wire be in
the vertical position, figured P' N', and it be carried in similar
directions, coinciding with the dotted horizontal curve so far, as to cut
the magnetic curves on the same side with it, the current will be from P'
to N'. If the wire be considered a tangent to the curved surface of the
cylindrical magnet, and it be carried round that surface into any other
position, or if the magnet itself be revolved on its axis, so as to bring
any part opposite to the tangential wire,--still, if afterwards the wire be
moved in the directions indicated, the current of electricity will be from