ly.
Inasmuch as a magnet has the power of attracting to itself pieces of
iron or steel, the basic importance of an electromagnet in telegraphy
will be at once apparent when we consider the sounder, whose clicks
are familiar to every ear. This instrument consists essentially of an
electro-magnet of horseshoe form with its two poles close together, and
with its armature, a bar of iron, maintained in close proximity to the
poles, but kept normally in a retracted position by a spring. When
the distant operator presses down his key the circuit is closed and a
current passes along the line and through the (generally two) coils of
the electromagnet, thus magnetizing the iron core. Its attractive power
draws the armature toward the poles. When the operator releases the
pressure on his key the circuit is broken, current does not flow, the
magnetic effect ceases, and the armature is drawn back by its spring.
These movements give rise to the clicking sounds which represent the
dots and dashes of the Morse or other alphabet as transmitted by the
operator. Similar movements, produced in like manner, are availed of
in another instrument known as the relay, whose office is to act
practically as an automatic transmitter key, repeating the messages
received in its coils, and sending them on to the next section of the
line, equipped with its own battery; or, when the message is intended
for its own station, sending the message to an adjacent sounder included
in a local battery circuit. With a simple circuit, therefore, between
two stations and where an intermediate battery is not necessary, a relay
is not used.

Passing on to the consideration of another phase of the phenomena of
electromagnetism, the reader's attention is called to Fig. 1, in which
will be seen on the left a simple form of electromagnet consisting of
a bar of soft iron wound around with insulated wire, through which a
current is flowing from a battery. The arrows indicate the direction of
flow.

All magnets have two poles, north and south. A permanent magnet (made of
steel, which, as distinguished from soft iron, retains its magnetism for
long periods) is so called because it is permanently magnetized and its
polarity remains fixed. In an electromagnet the magnetism exists only
as long as current is flowing through the wire, and the polarity of the
soft-iron bar is determined by the DIRECTION of flow of current around
it for the time being. If the direction is reverse