176 | - PHYSICS. 
thread, there dips a metallic bar, without touching the bottom. A horizonta, 
cross-beam is fastened to this metallic bar, ending in two balls, into which 
are inserted two bar magnets with their similar poles in the same direction. 
Another metallic bar is fixed’at right angles to the middle of the horizontal 
bar, ending in a bent point which dips into the mercury channel. Now, if 
one pole of the battery be dipped into the mercury cup, g, and the other intc 
the channel, the current will either pass from g through s, and from the upper 
end of the rod s into the channel, or it will move in the opposite direction. 
As soon as the current starts, the entire system, with the two bar magnets, 
begins to rotate about the axis formed by the thread. The direction of 
rotation depends partly upon which pole of the magnets is superior, and 
partly upon the direction of the current. 
The stand figured in pl. 22, figs. 45, 46, by the modification shown in 
jig. 47 may be also used to cause a magnet to rotate about its own axis. 
The wooden channel has here the same position as in fig. 45, the cork disk 
and magnet m, and stirrup b, only being moved. In their stead a bar magnet 
is suspended from a silk thread passing through the centre of the channel, a ° 
part of its length lying above, and a part below the plane of the channel. A 
socket screwed to the upper end of this magnet carries a mercury cup, ¢, in 
whose centre the thread is fastened by which the magnet is suspended. 
From a second socket which is screwed on the bar magnet at the level of 
the channel, there passes a metallic bar with a bent platinum point which dips 
into the mercury of the channel. As soon as one electrode of the battery is 
dipped into the mercury cup, ¢, and the other into the channel, the magnet 
commences to rotate about its axis. This rotation of a magnet about its 
own axis is explained by Ampére in the following manner: let abed (fig. 9, 
pl. 22) be the section of the magnet with the plane of the mercury, and let 
ab be one of the currents passing from the magnet through the mercury to 
the negative pole, then ab will be attracted by af, and ad repelled, so that 
the magnet must turn in a direction opposite to that of the currents of the 
magnet. In the figure, the curved arrows within the magnet indicate the 
direction of the current; those without, that of rotation. 
In pl. 22, fig. 10, let P be the centre of the vessel to whose circumference 
the current passes through the mercury. Let the shaded circle represent 
the section of the magnet, and the arrows surrounding it the direction of 
the currents forming the magnet. Considering the direction of the 
currents, PA, PA’, tangent to the magnet, the former produces a repulsion 
in the direction from m to c, the latter an attraction in the direction cm’. 
Both forces unite in a single one, acting in the direction from c to T’. 
Two other currents, as PB and PB’, one each side of the magnet, and at 
equal distances from it, likewise unite in a central force acting in the 
direction from c to T’. The magnet is thus impelled in a direction 
which is at right angles to cP, and must therefore continually rotate 
about P. 
Finally, one current may be set in rotation by another, as shown in 
pl. 20, fig. 87. The apparatus here figured consists of a copper vessel 
with an opening in the centre, through which passes a vertical metal rod, 
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