P .--.

FIG. 46.--Morse Signal Alphabet.

of the battery, in putting the line to one or the other, and thus
making the "dot" signal with the positive and the "dash" signal
with the negative pole. It follows that if the "dot" is indicated
by a throw of the needle to the right side, a "dash" will be given
by a throw to the left.

Most of the telegraph instruments for land lines are based on the
principle of the electro-magnet. We have already seen (page 59)
how Ampere found that a spiral of wire with a current flowing in
it behaved like a magnet and was able to suck a piece of soft iron
into it. If the iron is allowed to remain there as a core, the
combination of coil and core becomes an electro-magnet, that is to
say, a magnet which is only a magnet so long as the current
passes. Figure 47 represents a simple "horse-shoe" electro-magnet
as invented by Sturgeon. A U-shaped core of soft iron is wound
with insulated wire W, and when a current is sent through the
wire, the core is found to become magnetic with a "north" pole in
one end and a "south" pole in the other. These poles are therefore
able to attract a separate piece of soft iron or armature A. When
the current is stopped, however, the core ceases to be a magnet
and the armature drops away. In practice the electromagnet usually
takes the form shown in figure 48, where the poles are two bobbins
or solenoids of wire 61 having straight cores of iron which are
united by an iron bar B, and A is the armature.

Such an electromagnet is a more powerful device than a swinging