MAGNETISM AND ELECTRICITY. 179 
wrap a silk wound copper wire about a reel of wood or metal 
(pl. 22, fig. 18), whose inner cavity is large enough to receive a magnet, 
ab. The two extremities, m, n, of the coil are to be connected with the 
wires of a distant galvanometer. As soon as the magnet is inserted into 
the cavity of the reel, a deflection of the galvanometer ensues, which 
immediately ceases, to be renewed in the opposite direction, on the 
removal of the magnet. 
Pi. 22, fig. 19, illustrates an entirely different method of producing an 
electrical current by magnetism. Here ab is a strong horse-shoe magnet, 
mcn a horse-shoe electro-magnet, wrapped with a very long coil. Both 
extremities of the coil are connected with each other at a considerable 
distance. On quickly approximating the magnet, ab, to the legs of the 
horse-shoe, m,n, the magnetic fluid in the latter becomes decomposed, and a 
current arises in the coil which is demonstrated by its deflecting a simple 
magnetic needle, above or beneath which it passes. On removing the 
magnet the opposite deflection will be observed. By causing either the 
magnet or the electro-magnet to rotate rapidly about a vertical axis, so that 
the pole, m, which first stood over a, shall stand after a half revolution over 
b, and n over a, the coil will be continually traversed by currents whose 
directions alternate. : 
For conveniently examining the currents induced by magnetism, the 
magneto-electric rotating machines, as constructed by Pixii, Saxton, Clarke, 
Ettingshausen, and Stohrer, are very well adapted; in these, except the 
oldest of Pixii, the magnets are fixed. Pl. 22. fig. 20, exhibits one of these 
machines after the construction of Ettingshausen. A and B are the 
induction coils, wrapped round two cylinders of soft iron. The latter are 
fastened to the two ends of a horizontal iron plate, whose centre rests on a 
vertical iron axis, h (fig. 21). When this is rotated,the two cylinders pass 
under the poles of a very powerful battery composed of several horizontal 
horse-shoe magnets; in this manner each iron cylinder acquires an 
alternately north and south polarity. The coils on the two cylinders are 
wrapped from one wire of considerable length. One extremity of the wire 
is fastened by a screw to an iron ring, g (fig. 22), which is separated by a 
non-conductor from the iron axis of rotation, ; the other is similarly 
screwed to the iron plate carrying the two cylinders. On the iron axis of 
rotation another iron cylinder, /, is fastened, consisting of three divisions 
lying one above the other, the middle only of which has an uninterrupted 
circumference. In the upper part of h there are two channel-like 
depressions diametrically opposite to each other; at the lower end of ha 
segment is cut out, embracing about half the circumference. On each side 
of the axis of rotation is a small brass pillar with several apertures, provided 
with binding screws, in which metallic springs for closing the circuit may 
be inserted. Our figure represents the instrument as arranged to produce 
powerful physiological effects. In the two upper holes of the right pillar 
springs are screwed, one of which, during the rotation of the inductor (the 
entire rotating system), presses continually upon the iron ring, g, the other 
upon the upper surface of the cylinder, h. Consequently the circuit is 
ICONOGRAPHIC ENCYCLOPZDIA.—VOL. I. 23 353 
