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 position 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 current always being conformable to the law formerly expressed (114.), and also to the directions obtained with the rotating ball (101.).

174.  In these experiments it is not necessary to move the galvanometer or needle from its first position.  It is quite sufficient if the wire of the rectangle is distorted where it leaves the instrument, and bent so as to allow the moving upper part to travel in the desired direction.

175.  The moveable part of the wire was then arranged below the galvanometer, but so as to be carried across the dip.  It affected the instrument as before, and in the same direction; i.e. when carried from west to east under the instrument, the marked end of the needle went west, as before.  This should, of course, be the case; for when the wire is cutting the magnetic dip in a certain direction, an electric current also in a certain direction should be induced in it.

176.  If in fig. 31 dp be parallel to the dip, and BA be considered as the upper part of the rectangle (171.), with an arrow c attached to it, both these being retained in a plane perpendicular to the dip,—­then, however BA with its attached arrow is moved upon dp as an axis, if it afterwards proceed in the direction of the arrow, a current of electricity will move along it from B towards A.

177.  When the moving part of the wire was carried up or down parallel to the dip, no effect was produced on the galvanometer.  When the direction of motion was a little inclined to the dip, electricity manifested itself; and was at a maximum when the motion was perpendicular to the magnetic direction.