174 PHYSICS. 
water of the upper vessel. The lower vessel is connected with one pole of 
the battery, the rod ¢ with the other. Now, if the positive current enters 
the lower vessel it rises through the wires v and v’, descending again through 
the rod ¢; the system has, however, no directive power, since equal 
and opposite forces act on each wire. By taking one extremity of the 
inner wire from one or the other vessel, the current can ascend only 
through one wire, and the system, under the influence of terrestrial mag- 
netism, will place itself at right angles to the plane of the magnetic 
meridian. 
Two galvanic currents exert a magnetic influence on each other, 
attraction existing between two parallel currents when their direction is the 
same, and repulsion when this is opposite. The apparatus figured in fig. 4, 
pl. 22, is intended to illustrate these facts. Here abcdef is a copper 
rectangle, suspended in the mercury cups x and y. The galvanic current 
ascends through the post ¢, traverses the rectangle in the direction of the 
arrows, and descends along the post v. It is evident that the current in the 
post ¢ is the same in direction with that in the wire de, and that in v the 
same with that in bc. On bringing the rectangle out of this position, it will 
again return to it, owing to the attraction between ¢ and de, and between v 
and be. 
If a wire be doubled, as in fig.'70, pl. 22 (left hand), we have two currents 
which move in opposite directions to each other, and therefore produce no 
effect. That the action of a curved current is equal to that of a rectilineal 
of equal intensity, and whose length is equal to the direct distance between 
the extremities of the curved one, may be shown with the help of the wire 
represented on the right hand of fig. 70. This must be wrapped with silk 
to prevent any passage of galvanism from one wire to the other. On 
allowing a current to pass through the straight wire, which descends again 
through the bent one, this current will exert no influence on the rectangle 
( pl. 22, fig. 4), consequently the actions of the two wires must mutually 
balance each other. 
Two currents not parallel (crossed) exhibit a tendency to become parallel, 
and to move in the same direction; consequently, those parts of the current 
moving towards the crossing point attract, while one going and one 
returning repel each other. This may be shown by means of an apparatus 
exhibited sectionally by fig. 5, pl. 22, and in plan by fig.6. Two semi- 
circular channels made in a wooden disk are filled: with mercury and 
separated by insulating walls, a and 6. <A pivot point projects from the 
centre of the disk, upon which rests a copper needle, ed. with iron points ; 
a little below it lies another, ef, movable by hand, whose extremities are also 
of iron, and dip into the mercury. The current entering at x7 goes into one 
channel, then through both needles into the other, escaping at y. Giving 
the needles the position of p/. 22, fig. 6, the parts er and er repel each other, 
as also dr and fr. Bring them into such a position that the angle erd is less 
than 90°, and the above-mentioned parts attract each other. 
Ampére has propounded a very ingenious theory in explanation of these, 
phenomena. According to this savant, every particle of a magnet is encircled 
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