ESSENTIAL CONDITIONS OF MAGNETO-ELECTRIC INDUCTION. 185 
pole star, and then rotated so that its upper parts pass from west to east in 
conformity to the motion of the earth ; then positive electricity may be col- 
lected at the extremities of the magnet, and negative electricity at or about 
the middle of its mass. 
223. When the galvanometer was very sensible, the mere spinning of the 
magnet in the air, whilst one of the galvanometer wires touched the extremity, 
and the other the equatorial parts, was sufficient to evolve a current of elec- 
tricity and deflect the needle. 
224. Experiments were then made with a similar magnet, for the purpose 
of ascertaining whether any return of the electric current could occur at the 
central or axial parts, they having the same angular velocity of rotation as 
the other parts (259.) ; the belief being that it could not. 
225. A cylinder magnet, seven inches in length, and three quarters of an 
inch in diameter, had a hole pierced in the direction of its axis from one ex- 
tremity, a quarter of an inch in diameter, and three inches deep. A copper 
cylinder, surrounded by paper and amalgamated at both extremities, was fixed 
in the hole so as to be in metallic contact at the bottom, by a little mercury, 
with the middle of the magnet ; insulated at the sides by the paper; and pro- 
jecting about a quarter of an inch above the end of the steel. A quill was put 
over the copper rod, which reached to the paper, and formed a cup to receive 
mercury for the completion of the contact. A high paper edge was also raised 
round that end of the magnet, and mercury put within it, which however had 
no metallic connexion with that in the quill, except through the magnet itself 
and the copper rod (fig. 34.). The wires A and B from the galvanometer were 
dipped into these two portions of mercury; any current through them could, 
therefore, only pass down the magnet towards its equatorial parts, and then up 
the copper rod; or vice versa. 
226. When thus arranged and rotated screw fashion, the marked end of the 
galvanometer needle went west, indicating that there was a current through 
the instrument from A to B, and consequently from B through the magnet 
and copper rod to A (fig. 34.). 
227. The magnet was then put into a jar of mercury (fig. 35.) as before 
(219.) ; the wire A left in contact with the copper axis, but the wire B dipped 
in the mercury of the jar, and therefore in metallic communication with the 
mdcccxxxii. 2 B 
