In telegraphy an instrument for repeating the signals through a second
line. It is virtually a relay which is operated by the sender, and which
in turn operates the rest of the main line, being situated itself at
about the middle point of the distance covered. In the simpler forms of
repeater two relays are used, one for transmission in one direction the
other for transmission in the other. An attendant switches one or the
other in as required.

Thus a common relay is virtually a repeater for its local circuit. If
such a relay is placed half way down a line, and if the line beyond it
is connected as its local, it becomes a repeater.

Some forms of repeaters are automatic, and repeat both ways without the
need of an attendant.

It is the practice to somewhat prolong the signals sent through a
repeater.