MULTIPLE TRANSMISSION.
Although the printing and automatic
systems of telegraphing are used in America to some
extent, the larger part is done by the Morse system
of sound-reading and copying from it, either by pen
or the typewriter. In the early days only one
message could be sent over one wire at the same time,
but now from four to six or even more messages may
be sent over the same wire simultaneously without
one message interfering with the other. Like
most other inventions, many inventors have contributed
to the development of multiple transmission, till
finally some one did the last thing needed to make
it a success. The first attempts were in the
line of double transmission, and many inventors abroad
have worked on this problem.
Moses G. Farmer of Salem, Mass., proposed
it as early as 1852, and patented it in 1858.
Gintl, Preece, Siemens and Halske and others abroad
had from time to time proposed different methods of
double transmission, but no one of them was a perfect
success. When the line was very long there was
a difficulty that seemed insurmountable. In the
common parlance of telegraphy, there was a “kick”
in the instrument that came in and mutilated the signals.
About 1872 Joseph B. Stearns of Boston made a certain
application of what is called a “condenser”
to duplex telegraphy that cured the “kick,”
and from that time to this it has been a success.
Farther along I will tell you what occasioned this
“kick” and how it was cured. If this
or some other method could be applied as successfully
to cure the many chronic “kickers” in the
world it would be a great blessing to mankind.
It has always been a mystery to the
uninitiated how two messages could go in opposite
directions and not run into one another and get wrecked
by the way. If you will follow me closely for
a few minutes I will try to tell you.
We have already stated that an electromagnet
is made by winding an insulated wire around a soft
iron core. If we pass a current of electricity
through this wire the core becomes magnetic, and remains
so as long as the current passes around it. In
duplex telegraphy we use what is called a differential
magnet. A differential electromagnet is wound
with two insulated wires and so connected to the battery
that the current divides and passes around the iron
core in opposite directions. Now if an equal
current is simultaneously passed through each of the
wires of the coil in opposite directions the effect
on the iron will be nothing, because one current is
trying to develop a certain kind of polarity at each
pole of the magnet, while the current in the other
wire is trying to develop an opposite kind in each
pole. There is an equal struggle between the
two opposing forces, and the result is no magnetism.
This assumes that the two currents are exactly the
same strength.
If we break the current in one of
the coils we immediately have magnetism in the iron;
or if we destroy the balance of the two currents by
making one stronger than the other we shall have magnetism
of a strength that measures the difference between
the two.