MAGNETISM¢ AND: ELECTRICITY. 1605 
body. it becomes necessary to employ series of many pairs. By connecting 
the positive poles of several elements, and likewise the negative, we obtain 
the equivalent of a single pair or element of greater surface. Pl. 22, fig. 54, 
illustrates this combination. Here A is an element closed by the wire abc, 
B is a second element; the positive poles of both are united at a, and 
the negative at c. 
D. Execrro-Macnetism. 
a. Magnetic Action of the Galvanic Current. 
We shall now proceed to treat more in detail of the magnetic actions of 
the galvanic current. The most important of these consists in its being able 
to render iron, steel, and even other metals, magnetic. Wind a copper wire 
spirally round a glass tube, and within this lay a fine sewing needle. If, now, 
a galvanic current be passed through the wire for a short time, the needle 
will become permanently magnetic. In right-handed spirals or coils 
(pl. 20, fig. 76), where the turns are as in the common screw, the north 
end of the needle will be where the positive current enters; in left-handed 
( fig.77), where it emerges. If, on the same tube, the wire be wound alter- 
nately right and left (fig. 78), several successive poles will be formed in the 
needle. In this manner magnets of extraordinary power may be obtained 
from soft iron. For this purpose. a strong piece of iron bent into the horse- 
shoe form is to be wrapped round with insulated copper wire ( pl. 20. fig. 79). 
The wire must be wound in the same direction on both legs. If the winding 
be right-handed, then the north pole will be where the positive current enters, 
as at a, the south pole being at b. A single pair of plates of large surface is 
generally used with this electro-magnet. If several small elements are 
employed, a greater number of windings will be required. Figs. 80 and 81, 
pl. 20, represent a poweriul electro-magnet, capable of sustaining over 
2000 lbs. It consists of two cylindrical pieces of iron, each about three and 
a half inches thick, and from two to two and a half feet long, bent into the 
horse-shoe form; both arms are wrapped with a copper wire, insulated by 
being covered with silk, about three thousand feet long and one fortieth of an 
inch thick. The galvanic series producing the current consists of thirty- 
four pairs of plates. When the current begins to circulate, the lower 
movable electro-magnet, a’b’, is attracted by the upper, and both are 
attached so firmly that the immense weight of one ton may be Jaid upon the 
board, cc, without separating the electro-magnets. The honor of first 
applying the principle of the electro-magnet to the production of very large 
magnets, is due to Prof. Joseph Henry. 
On account of the powerful magnetic action of the galvanic current, the 
idea early presented itself of using electro-magnetism as a motive power. 
Instruments for this purpose are called electro-magnetic machines. PJ. 22, 
jigs. 36-38, represent one form of the electro-magnetic machine as con- 
structed by Stohrer of Leipzig. in 1841: the action of this depends upon 
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