MAGNETISM AND ELECTRICITY. 181 
wires, n and o, dip into mercury cups. Jig.57 shows the method of 
charging a Leyden jar. Wind a piece of copper wire about the external 
coating of the jar, and connect it with the lower part of the magneto-electric 
machine ; remove the sponge from the director, m, and connect its wire with 
the extremity of the intensity inductor; rotate the inductor with moderate 
velocity, hold the director by the wooden handle, and let it touch for a 
moment the knob of the jar until a single spark passes over. On bringing 
the knob of the jar into connexion with a sensitive gold-leaf electrometer, the 
latter will indicate a feeble charge of the jar. Fig. 58 shows the manner in 
which the magneto-electric machine can produce rotation. Here b’b’ 1s 
a vertical horse-shoe magnet on a tripod stand, a’; d’ is a connecting fork ; 
e'f’ two wire frames with mercury cups above. On pouring mercury into 
the large vessel, and arranging the wires as in the figure, an uninterrupted 
rotation will be produced. 
Figs. 23-27, pl. 22, represent the magneto-electric machine of Stdhrer, 
which, instead of a single magnetic battery, contains three, standing verti- 
cally. Quite recently Stéhrer has constructed machines of still larger size. 
one of them for the university of Dorpat ; in these, however, the magnets 
are horizontal. 
c. Rotation Magnetism. 
It still remains to mention the so-called rotation magnetism. Arago 
discovered that when a horizontal copper disk is rapidly rotated under a 
delicately suspended magnetic needle, the latter turns in the same direction 
about its axis. In his experiments he made use of the apparatus shown in 
pl. 22, figs. 52, 53, and 53%. In fig. 52, h is a clock-work constructed 
entirely of copper or brass, excepting some steel pivots; this stands on a 
firm wooden tripod, and is intended to communicate a rapid rotation to a 
vertical axis, x (fig. 53). The latter communicates its motion to a piece of 
brass, tt, separately figured in fig. 53¢, on which the copper disks to be 
employed are fastened. Three vanes on the above-mentioned piece of brass 
are intended to regulate the velocity of rotation by their greater or less incli- 
nation. <A four-footed table, pp’ (fig. 52), 1s set over the clockwork, having 
an opening in the middle somewhat greater than the rotating disk, but pasted 
over beneath with a piece of paper. <A glass bell,c (fig. 53), is laid on the 
table, in which the magnetic needle, gg’, is suspended by a silk thread, f; 
the magnet may be raised or lowered at pleasure by the turning of a small 
axle. The rotation of the disk may also be effected without clockwork. 
The deflecting force in the rotation of the disk increases with the velocity, 
but decreases with the distance of the disk from the needle. In disks of 
other metals than copper, as tin, lead, or zinc, the action is much feebler ; 
it is feeblest with bismuth. It is to Faraday that we owe the explanation 
of these phenomena by the theory of induced currents. 
355 
