Telegraph, Multiplex.
A system of telegraphy by which a number of messages can be transmitted
in both directions over a single wire. The principles underlying the
systems are the following:
Suppose that at the two extremities of a telegraph line two arms are
kept in absolute synchronous rotation. Let the arms in their rotation,
press upon as many conducting segments as there are to be transmissions
over the line. A transmitting and receiving set of instruments may be
connected to one segment at one end of the line, and another set to the
corresponding segment at the distant station. For each pair of segments
two sets can be thus connected. Then if the arm rotates so rapidly that
the contacts succeed each other rapidly enough each pair of sets of
instruments can be worked independently of the others. In practice this
rapid succession is effected by having a number of contacts made for
each pair during a single rotation of the arm or equivalent.
The multiplex system has been perfected by the use of La Cour's phonic
wheel (see Phonic Wheel), and brought into a practical success by
Patrick B. Delany, of New York.
Two phonic wheels rotate at each end of the line. They are kept in
synchronous motion by two vibrating steel reeds of exactly the same
fundamental note, and the axle of each wheel carries an arm whose end
trails over the contacts or distributor segments already spoken of. The
reeds are adjusted to vibrate at such speed that the trailer is in
contact with each segment about 1/500 second. The number of groups of
segments required for each working is determined by the retardation of
the signals owing to the static capacity of the line. To convert the
rapidly recurring impulses of current into practically a single current,
condensers are connected across the coils of the relay. One battery
serves for all the arms.
Multiplex telegraphy can effect from two to six simultaneous
transmissions over one wire. For two or four transmissions the method
only distinguishes it from duplex or quadruplex telegraphy. The terms
diode, triode, tetrode, pentode and hexode working are used to indicate
respectively the simultaneous transmission of two, three, four, five, or
six messages over one wire.
It will be seen that the multiplex process really assigns to each
transmission separate times, but divides these times into such short and
quickly recurring intervals that the work is executed as if there was
continuous contact. In no case is there the popular conception of the
sending of several messages actually simultaneously over one wire. Each
signal in reality has its own time assigned it, divided into short
periods of high frequency, and only utilizes the line when it is free.