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

SINGLE-NEEDLE INSTRUMENT AND VOLTAIC CURCUIT. 223
the mechanism of the instrument has been considered, I will
repeat, in part, and extend that description to the operation
in connection with the voltaic battery.
The bobbins or coils A, are made Fig. 7.
of very fine insulated copper wire,
in size about T-1 — of an inch in di-
ameter, or about No. 36, American gauge. These coils are
from two to three inches lono' | i,' \
in the form as seen by the differ- |
ent figures. The interior needle Vl'
is in the rhomboid form, one and,
an eighth inch long and seven d
eighths of an inch broad. Some- | ) i
tinmes several magnetized short
needles are substituted for the \ )
one, all firmly secured on either
or both sides of a thin ivory disk.
The index or exterior needle,
seen in fig. 6, is about thrce ei
inches long. The frame of the " - " I
coils x is made of copper, wood,
ivory, or of any other material. This frame is screwed to a plate of copper, on the sides
of the telegraph instrument. The wires surrounding the
right hand bobbin or coil is fastened to the screw G, as seen
in fig. 7, which, by means of a metallic strap, is connected
with the c on the right of the figure, secured on the base of the
apparatus. The other end of the wire, on the left hand bobbin
or coil, is in contact with another screw, Dsupported by a
strip of brass, which is fixed to the base; from this brass plate
there rises an upright stiff steel spring d, which presses strongly
against a point attached to an insulated brass rod r, screwed
against the side of the case; on the opposite side of this rod is
another point, against which a second steel spring d presses, and
this spring is attached to a brass plate E, terminated by the
binding-screw E'; this binding-screw E' is the terminal of the
wire from the left hand coil. If c on the right, and E/ on the
left, be connected by a wire, w, the current will flow from c,
on the right of the figure, through G, into the right-hand coil,
out from the left-hand coil to D, thence through d r d to E, and
to the terminal screw E/, and around the wire circuit vw w,
back to c on the right of the figure. The battery contact is
broken, and the direction of the current reversed, by the action
of the spring d cl, in the following manner