The telegraph manual: a complete history and description of the semaphoric, electric and magnetic telegraphs of Europe, Asia, Africa, and America, ancient and modern.

ELECTRO-MAGNET OF 1844. 445
side a surface clean and smooth, and parallel with the faces,
F and F.
The coils or helices of wire which surround the prongs A A,
necessary to complete the electro-magnet, consist of many
turns of wire, first running side by side, covering the form
upon which the spiral is made, until the desired length of the
coil is obtained; the wire is then turned back, and wound
upon the first spiral, covering it, until the other end of the coil
is reached, where the winding began; then again mounting
upon the second spiral, covers it, and in the same manner it is
wound back and forth, until the required size of the coil is
attained.
The coil is wound upon a form of the size (or a little larger)
of the legs of the magnet, and when the coil is completed, the
form is taken out, leaving an opening in the centre, B, into
which the prongs may freely pass. Fig. 32 represents a coil
constructed in the manner described. A and A are the two
ends of wire which are brought out from the coils. The one
proceeds from the centre of the coil, and the other from the
outside. c and c are circular wooden heads, on each end of
the coil, and fastened to it by binding wire, running from one
head to the other around the coil. The wire used in constructing it, as heretofore mentioned, is covered in the same manner
as bonnet wire, and saturated or varnished with gum shellac.
This preparation is considered necessary, in order to prevent a
metallic contact of the wires with each other. Such a contact
of some of the wires with others encircling the iron prong
would either weaken or altogether destroy the effect intended
by their many turns. If the wires were bare instead of being
covered, the electric fluid, when applied to the two ends, A and
A, instead of passing through the whole length of the wire in
the coil as its conductor, would pass laterally through it as a
mass of copper, in the shortest direction it could take. For this
reason they require a careful and more perfect insulation.
Two coils are thus prepared for each magnet, one for each
prong A and A, fig. 31."
Such was the construction of the magnets in 1844. The
wire was large, and one pair of coils weighed 185 pounds.
Since then the ingenious spirit of the age has reduced the size
and weight; the usual weight does not exceed from one to two
pounds; the wire is very fine, and well covered or insulated
with silk. The mechanism has very much changed; so much
so, in fact, that the telegrapher unacquainted with the facts in
the case, would not suppose the magnets above described ever
belonged to the telegraph.