MAGNETISM AND ELECTRICITY. 173 
figure, the positive current passes from b to b’, or the reverse, as b or b’ is 
positive or negative. To restore one of the two above-mentioned 
connexions at pleasure, the wooden swing-beam represented in pl. 20, fig. 85, 
may be employed; this turns on an axis, aa’, and carries four bent 
conductors, d, d', e, e’. The two former are elevated in the figure, the two 
latter connect 7 with v. and 7’ with ¢; when e and e’ are elevated, 7 is 
connected with t by d, and r’ with v’ by d’. This apparatus, termed 
gyrotrope, is attached at the feet of the pillars, v and ¢, of fig. 83, these 
being united by the strips b and b’. Taking now a copper wire bent into a 
circular form, and immersing its steel-pointed extremities in the mercury 
cups, « and y, of fig. 83, the wire will turn and arrange itself with its plane 
forming a right angle with the magnetic meridian, the positive current 
passing in the Jower half of the circle from east to west. Reverse the 
current by means of the gyrotrope, and the circle will make a semi- 
revolution. The result will be precisely the same with a wire bent at right 
angles. A combination of several circular wires, parallel to each other and 
traversed by the current in the same direction, places itself, like a single 
circular current, at right angles to the magnetic meridian. Therefore the 
spiral wire (pl. 22, fig. 8), when suspended to an Ampére stand, and 
traversed by a current, must so place itself that the axis of the spiral shall 
fall in the direction of the declination compass, so that the latter may be 
imitated by such a spiral. 
The apparatus of De la Rive (pl. 20, fig.88) shows that even feeble currents 
are thus affected by magnetism. ‘T'wo plates, one of zinc, the other of copper, 
are fastened to a piece of cork, and united above the cork by a copper wire 
wound either circularly or as in the figure. On placing the cork on slightly 
acidulated water, a galvanic current is immediately produced, strong 
enough to be directed by terrestrial magnetism, or to be attracted and 
repelled by a magnet. ‘To examine the influence of a magnet on a galvanic 
current entirely free from the complication of terrestrial magnetism, an appa- 
ratus must be employed in which the influence of terrestrial magnetism neu- 
tralizes itself, as is the case in the double rectangle shown in pl. 20, fig. 86. 
Here a wire, symmetrical on both sides of the axis of rotation, is traversed 
by the galvanic current in one direction. On suspending such a rectangle 
from the Ampére stand, it remains in equilibrium in all directions, but is 
attracted or repelled by the poles of a magnet. 
It is necessary to distinguish between the action of terrestrial magnetism 
on vertical and on horizontal currents. For vertical currents we may 
make use of the apparatus represented in pl. 22, fig. 1, which consists of 
two cylindrical copper vessels filled with acidulated water, the lower 
cylinder having rather the greater diameter. Both have a cylindrical 
aperture in the centre, through which passes a rod, ¢, whose upper extremity 
forms a mercury cup. The cross-piece, hh’, of some non-conducting material, 
has a pivot point in its middle; on this point it rests in the bottom of 
the mercury cup, and is capable of free rotation. The lower extremities of 
the wires vv’ dip into the fluid of the lower vessel; above, after some 
windings, they are fastened on the cross-piece, hh’, and then dip into the 
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