TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 
171 
afforded, render this single experiment an epitome of nearly all the facts of 
magneto-electric induction. 
171. A piece of common copper wire, about eight feet long and one twen¬ 
tieth of an inch in thickness, had one of its ends fastened to one of the termi¬ 
nations of the galvanometer wire, and the other end to the other termination ; 
thus it formed an endless continuation of the galvanometer wire: it was then 
roughly adjusted into the shape of a rectangle, or rather of a loop, the upper 
part of which could be carried to and fro over the galvanometer, whilst the 
lower part, and the galvanometer attached to it, remained steady (Plate IV. 
fig. 30.). Upon moving this loop over the galvanometer from right to left, 
the magnetic needle was immediately deflected ; upon passing the loop back 
again, the needle passed in the contrary direction to what it did before; upon 
repeating these motions of the loop in accordance with the vibrations of the 
needle (39.), the latter soon swung through 90° or more. 
172. The relation of the current of electricity produced in the wire, to its 
motion, may be understood by supposing the convolutions at the galvanometer 
away, and the wire arranged as a rectangle, with its lower edge horizontal 
and in the plane of the magnetic meridian, and a magnetic needle suspended 
above and over the middle part of this edge, and directed by the earth (fig. 30.). 
On passing the upper part of the rectangle from west to east into the posi¬ 
tion represented by the dotted line, the marked pole of the magnetic needle 
went west; the electric current was therefore from north to south in the 
part of the wire passing under the needle, and from south to north in the 
moving or upper part of the parallelogram. On passing the upper part of the 
rectangle from east to west over the galvanometer, the marked pole of the 
needle went east, and the current of electricity was therefore the reverse of the 
former. 
173. When the rectangle was arranged in a plane east and west, and the 
magnetic needle made parallel to it, either by the torsion of its suspension 
thread or the action of a magnet, still the general effects were the same. On 
moving the upper part of the rectangle from north to south, the marked pole 
of the needle went north ; when the wire was moved in the opposite direction, 
the marked pole went south. The same effect took place when the motion of 
the wire was in any other azimuth of the line of dip ; the direction of the cur- 
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