message, and at the post on the right it is conformably in the
position for receiving the message. The key is so constructed that
when it is at rest it puts the line in connection with the earth
through the RECEIVING INSTRUMENT and the earth plate.

The key K consists essentially of a spring-lever, with two
platinum contacts, so placed that when the lever is pressed down
by the hand of the telegraphist it breaks contact with the
receiver R, and puts the line-wire L in connection with the earth
E through the battery B, as shown on the left. A current then
flows into the line and traverses the receiver R' at the distant
station, returning or seeming to return to the sending battery by
way of the earth plate E' on the right and the intermediate
ground.

The duration of the current is at the will of the operator who
works the sending-key, and it is plain that signals can be made by
currents of various lengths. In the "Morse code" of signals, which
is now universal, only two lengths of current are employed--
namely, a short, momentary pulse, produced by instant contact of
the key, and a jet given by a contact about three times longer.
These two signals are called "dot" and "dash," and the code is
merely a suitable combination of them to signify the several
letters of the alphabet. Thus e, the commonest letter in English,
is telegraphed by a single "dot," and the letter t by a single
"dash," while the letter a is indicated by a "dot" followed after
a brief interval or "space" by a dash.

Obviously, if two kinds of current are used, that is to say, if
the poles of the battery are reversed by the sending-key, and the