oil is on a line with
the needle--in fact, covers it. Next touch the terminals of a battery
with the ends of the wire. The needle at once shifts either to right or
left, and remains in that position as long as the current flows. If you
change the wires over, so reversing the direction of the current, the
needle at once points in the other direction. It is to this conduct on
the part of a magnetic needle when in a "magnetic field" that we owe the
existence of the needle telegraph instrument.

NEEDLE INSTRUMENTS.

[Illustration: FIG. 54.--Sketch of the side elevation of a Wheatstone
needle instrument.]

Probably the best-known needle instrument is the Cooke-Wheatstone,
largely used in signal-boxes and in some post-offices. A vertical
section of it is shown in Fig. 54. It consists of a base, B, and an
upright front, A, to the back of which are attached two hollow coils on
either side of a magnetic needle mounted on the same shaft as a second
dial needle, N, outside the front. The wires W W are connected to the
telegraph line and to the commutator, a device which, when the operator
moves the handle H to right and left, keeps reversing the direction of
the current. The needles on both receiving and transmitting instruments
wag in accordance with the movements of the handle. One or more
movements form an alphabetical letter of the Morse code. Thus, if the
needle points first to left, and then to right, and comes to rest in a
normal position for a moment, the letter A is signified;
right-left-left-left in quick succession = B; right-left-right-left = C,
and so on. Where a marking instrument is used, a dot signifies a "left,"
and a dash a right; and if a "sounder" is employed, the operator judges
by the length of the intervals between the clicks.

INFLUENCE OF CURRENT ON A MAGNETIC NEEDLE.

[Illustration: FIGS. 55, 56.--The coils of a needle instrument. The
arrows show the direction taken by the current.]

Figs. 55 and 56 are two views of the coils and magnetic needle of the
Wheatstone instrument as they appear from behind. In Fig. 55 the current
enters the left-hand coil from the left, and travels round and round it
in a clockwise direction to the other end, whence it passes to the other
coil and away to the battery. Now, a coil through which a current passes
becomes a magnet. Its polarity depends on the direction in which the
current flows. Suppose that you are looking through the coil, and that
the current enters