The key by which the messages are transmitted is shown in Fig. 319. M is
a base plate of brass. A is a brass lever, mounted on an arbor G carried
between adjustable set screws D. C is the anvil where contact is made by
depressing the key by the finger piece B of ebonite. E, Fl are adjusting
screws for regulating the vertical play of the lever. H is the switch
for opening or closing the circuit. It is opened for transmission, and
closed for receiving. By screws, L L, with wing nuts, K K, the whole is
screwed down to a table.
Fig. 319. MORSE TELEGRAPH KEY.
In the United States the simplest disposition of apparatus is generally
used. The main line is kept on closed circuit. In it may be included a
large number of relays at stations all along the line, each with its own
local circuit. There may be fifty of such stations. Battery is generally
placed at each end of the line. Very generally gravity batteries are
used, although dynamos now tend to supplant them in important stations.
As relays the ordinary relay is used. Its local circuit includes a
sounder and local battery. The latter is very generally of the gravity
type, but oxide of copper batteries (See Battery, Oxide of Copper) are
now being introduced. At main or central offices, the terminals of the
lines reach switch boards, where by spring-jacks and plugs, any desired
circuits can be looped into the main circuit in series therewith.
In European practise the main line is kept on open circuit. Polarized
relays are used to work the local circuits. The currents for these have
to be alternating in direction. When the line is not in use its ends are
connected to earth at both ends, leaving the battery out of circuit.
Each intermediate station has its own main, or line battery for use when
it desires to send a message. In the American system as first described,
it will be seen that the main batteries are at most two in number.
For the details of the different apparatus, the following definitions
may be consulted: Embosser, Telegraphic--Recorder, Morse--Relay--Relay
Connection--Sounder.