MAGNETISM AND ELECTRICITY. 1765 
by acircular electric current returning into itself. The transverse section of 
a magnetic bar will then be something as in pl. 22, fig. 48, although a magnetic 
bar may also be considered as a system of parallel closed currents, as shown 
in fig.44. Let us imagine a wire helix extending from m (pl. 20, jigs. 89, 90) 
in both directions, and traversed by a current in the direction of the arrows ; 
let us further suppose this helix to be severed at m, and both parts separated, 
then there will be produced a south pole at @ anda north pole at b, both 
attracting each other. Circulating currents may be imagined even in the 
interior of the earth, which are parallel to the magnetic equator; instead of 
these, however, we may suppose a single current, the mean terrestrial 
current, which passes from east to west, lying, for each place, in a plane 
perpendicular to the. dipping needle. The latter may be shown by the 
apparatus, pl. 22, fig.7. If this be placed with the horizontal axis of 
rotation perpendicular to the magnetic meridian, then the plane in which 
the rectangular current places itself in equilibrium, must be parallel to the 
plane of the terrestrial current; the experiment, however, shows that this 
’ qs exactly perpendicular to the direction of the dipping needle. 
Ampére’s theory also explains the rotation of a movable current about a 
magnet, as is shown by the apparatus figured in pl. 22, fig.45. A horizontal 
_ bar, d, may be moved up and down the vertical rod /, and fixed at any posi- 
tion. The bar d carries a brass ring, on which is set a wooden channel for 
holding mercury. In this is stuck a cork disk, through whose centre passes 
a vertical magnetic bar, mm, at whose upper extremity is screwed a socket 
with a steel mercury cup, p. A fine pivot rotating in the cup carries a 
copper stirrup, b, which is bent down at both ends, and whose platinum- 
pointed extremities dip into the mercury channel; in its centre is a second 
mercury cup. On dipping one electrode of a battery into this cup, and the 
other into the mercury channel, the current will traverse both arms of the 
copper stirrup, which will begin.to rotate about the magnet. 
Another apparatus, invented by Faraday, which begets its own current, 
thus dispensing with a battery, is shown in pl. 22, fig. 3. Here zz is a vessel 
of zinc containing acidulated water, and perforated in the centre; above the 
centre of the aperture a cross-piece of zinc is laid, and to it is fastened a 
copper rod, sc, ending above in a mercury cup. From this cup is suspended 
the apparatus shown in fig. 2, the lower part being a ring of copper. The 
positive current here passes from the zinc through the acidulated water into 
the copper ring, then ascends through the wires, and descends again through 
the copper rod cs into the zinc. A rapid rotation will immediately ensue 
whenever a magnet is brought under the vessel. Fig. 3 represents the 
arrangement of fig. 2 as set in the mercury cup cs. 
For the same reason a movable magnet will rotate about a fixed immovable 
current. ‘T’o prove this it is only necessary to make a slight alteration in 
the apparatus shown in pl. 22, fig. 45. For this purpose, remove the cork 
disk with the magnetic bar m, and the copper stirrup 0, and fix the horizontal . 
beam, d, in such a position that the upper extremity of the copper rod, s, 
shall be exactly opposite the centre of the wooden channel ( fig.46). At 
this upper extremity there is a mercury cup, into which, suspended by a 
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