ESSENTIAL CONDITIONS OF MAGNETO-ELECTRIC INDUCTION. 183 
rotated near it; for I have found by careful experiment, that when a constant 
current of electricity was sent successively through a series of wires of cop- 
per, platina, zinc, silver, lead, and tin, drawn to the same diameter ; the 
deflection of the needle was exactly equal by them all. It must be remem- 
bered that when bodies are rotated in a horizontal plane, the magnetism of 
the earth is active upon them. As the effect is general to the whole of the 
plate, it may not interfere in these cases ; but in some experiments and calcu- 
lations may be of important consequence. 
217. Another point which I endeavoured to ascertain, was, whether it was 
essential or not that the moving part of the wire should, in cutting the mag- 
netic curves, pass into positions of greater or lesser magnetic force ; or whe- 
ther, always intersecting curves of equal magnetic intensity, the mere motion 
was sufficient for the production of the current. That the latter is true, has 
been proved already in several of the experiments on terrestrial magneto-elec- 
tric induction. Thus the electricity evolved from the copper plate (149.), the 
currents produced in the rotating globe (161, &c.), and those passing through 
the moving wire (1 71. )> are all produced under circumstances in which the 
magnetic force could not but be the same during the whole experiment. 
218. To prove the point with an ordinary magnet, a copper disc was 
cemented upon the end of a cylinder magnet, with paper intervening ; the 
magnet and disc were rotated together, and collectors (attached to the galva- 
nometer) brought in contact with the circumference and the central part of the 
copper plate. The galvanometer needle moved as in former cases, and the 
direction of motion was the same as that which would have resulted, if the cop- 
per only had revolved, and the magnet been fixed. Neither was there any 
apparent difference in the quantity of deflection. Hence, rotating the magnet 
causes no difference in the results ; for a rotatory and a stationary magnet pro- 
duce the same effect upon the moving copper. 
219. A copper cylinder, closed at one extremity, was then put over the 
magnet, one half of which it inclosed like a cap ; it was firmly fixed, and pre- 
vented from touching the magnet anywhere by interposed paper. The ar- 
rangement was then floated in a narrow jar of mercury, so that the lower edge 
of the copper cylinder touched the fluid metal ; one wire of the galvanometer 
dipped into this mercury, and the other into a little cavity in the centre of the 
