TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 
167 
by several arrangements. Weak as the current may seem to be, it is as strong 
as, if not stronger than, any thermo-electric current; for it can pass fluids (23.), 
agitate the animal system, and in the case of an electro-magnet has produced 
sparks (32.). 
155. A disc of copper, one fifth of an inch thick and only one inch and a 
half in diameter, was amalgamated at the edge ; a square piece of sheet lead, 
(copper would have been better) of equal thickness had a circular hole cut in 
it, into which the disc loosely fitted ; a little mercury completed the metallic 
communication of the disc and its surrounding ring ; the latter was attached 
to one of the galvanometer wires, and the other wire dipped into a little me- 
tallic cup containing mercury, fixed upon the top of the copper axis of the 
small disc. Upon rotating the disc in a horizontal plane, the galvanometer 
needle could be affected, although the earth was the only magnet employed, 
and the radius of the disc but three quarters of an inch; in which space only 
the current was excited. 
156. On putting the pole of a magnet under the revolving disc, the galva- 
nometer needle could be permanently deflected. 
157- On using copper wires one sixth of an inch in thickness instead of the 
smaller wires (86.) hitherto constantly employed, far more powerful effects 
were obtained. Perhaps if the galvanometer had consisted of fewer turns of 
thick wire instead of many convolutions of thinner, more striking effects would 
have been produced. 
158. One form of apparatus which I purpose having arranged, is to have 
several discs superposed ; the discs are to be metallically connected, alternately 
at the edges and at the centres, by means of mercury ; and are then to be re- 
volved alternately in opposite directions, i. e. the first, third, fifth, &c. to the 
right hand, and the second, fourth, sixth, &c. to the left hand ; the whole being 
placed so that the discs are perpendicular to the dip, or intersect most directly 
the magnetic curves of powerful magnets. The electricity will be from the 
centre to the circumference in one set of discs, and from the circumference to 
the centre in those on each side of them ; thus the action of the whole will 
conjoin to produce one combined and more powerful current. 
159. I have rather, however, been desirous of discovering new facts and 
new relations dependent on magneto -electric induction, than of exalting the 
