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.
Without specifically describing here the entire mechanism--since this is
not a text-book or a treatise--we may say that a duplex telegraph-line
is fitted with these differentially wound electromagnets at every
station. When Station A (Fig. 3) is connected to the line by the
positive pole of its battery, Station B will have its negative pole to
line and its positive to earth. When A depresses his key to send a
message, half the current passes by one set of coils around his
differential magnet through a short resistance-coil to the earth, and
the other half by the contrary coil around the magnet to the line, and
so to Station B. The divided current does not affect A's own station,
being neutralized by the differential magnet, but it does affect B,
whose instrument responds and gives him the message.
Now B may at the same time send a message to A by half of his own
divided current from his own end of the line.
[Illustration: Fig. 3.
Represents a duplex 500-mile telegraph-line. A and B are the two
terminal stations; B B', the batteries; K K', the keys; D D',
the small resistance-coils, equal to the battery-resistance when
the latter is not in circuit; R R', resistances each equal to
the 500-mile line; and C C', condensers giving the artificial
lines R R' the same capacity as the 500-mile line.]
The puzzle to most people is: How can the signals pass each other in
different directions on the same wire? But the signals do not have to
pass each other. In effect, they pass; but in fact, it is like going
round a circle--the earth forming half. A sends his message over the
line to B. B sends his message to A through the earth and up A's
ground-wire. The operative who is sending with positive pole to line
_pushes_ his current through--so to speak--while the operative who is
sending with the negative pole to line _pulls_ more current in the same
direction through the line whenever he closes his key.
This may not be a strictly scientific statement; but, as long as we
speak of a "current" flowing from positive to negative poles (which is
the invariable course electricity takes), it is the way to look at the
matter understandingly.
The short "resistance-coil" at each end, fortified by a "condenser" made
of many leaves of isolated tin-foil, to give it capacity, offers
precisely the same resistance to the current as the 500 miles of wire
line; so that the twin currents that run around the differential magnet
exactly neutralize each other and make no effect in the office the
message starts from; while one of them takes to the earth, and the other
to the line to carry the message.
This condenser is necessary, because the short resistance-coil affects
the current immediately, while the long line with its greater amount of
metal does not give the same amount of resistance till it is filled from
end to end, which requires a fraction of a second. During this time,
however, more current is passing through the differential coil connected
with the line than through the short resistance-coil; and the unequal
flow causes the relay armature to jump, or "kick." The condenser, with
the many leaves of tin-foil, supplies the greater metal surface to be
traversed by the short line current, causes the flow to be equal in both
circuits at all times, and thus cures the "kick." It is this quality of
a condenser that enables us to give to an artificial line of any
resistance all the qualities, including capacity, and exhibit all the
phenomena of a real line of any length, and it was this quality that
enabled Mr. Stearns to take the "kick" out of duplex transmission and
thus change the whole system, which created a new era in telegraphy.
We have just spoken of the "capacity" of a circuit, and stated that it
was determined by the mass of metal used. This capacity is measured by a
standard of capacity that is arbitrary and consists of a condenser,
constructed so that a given amount of surface of tin-foil may be plugged
in or out. The practical unit of capacity is called the micro-farad, the
real unit is the farad, and takes its name from Faraday.
But let us go back to multiple systems of transmission. There are many
other systems of simultaneous transmission aside from the duplex, and
all of them are classed under the general head of multiple telegraphy.
First there is the quadruplex, that sends two messages each way
simultaneously, making one wire do the work of four single wires--as
they were used at first. The quadruplex is very extensively used by the
Western Union Telegraph Company and others. It would be difficult to
explain it in a popular article, so we will not attempt it. There is
another form of multiple telegraph that was used on the Postal Telegraph
line when it first started--which was invented and perfected by the
writer--that can be more easily explained.
In 1874 I discovered a method of transmitting musical tones
telegraphically, and the thing that set my mind in that direction was a
domestic incident. It is a curious fact that most inventions have their
beginnings in some incident or observation that comes within the
experience of some one who is able to see and interpret the meaning of
such incidents or observations. I do not mean to say that inventions are
usually the result of a happy thought, or accident; the germ may be, but
the germ has to have the right kind of soil to take root in and the
right kind of culture afterward. It is a rare thing that an invention,
either of commercial or scientific importance, ever comes to perfection
without hard work--midnight oil and daylight toil; and it is rarely, if
ever, that a discovery or an invention based upon a discovery does not
have, sooner or later, a practical use, although we sometimes have to
wait centuries to find it put. We had to wait forty-four years after
the galvanic battery was discovered before it became a useful servant of
man. It was fifty years or more after the discovery by Faraday of
magneto-electricity before it found a useful application beyond that of
a mere toy, but now it is one of the most useful servants we have, as
shown in its wonderful development in electric lighting and electric
railroads, to say nothing of its heating qualities and the useful
purpose it serves in driving machinery. The interesting discoveries of
Professor Crookes in passing a current of electricity through tubes of
high vacua waited many years before they found a practical use in the
X-ray, that promises to be of great service in medicine and surgery.
The transmission of musical harmonies telegraphically, while in itself
of great scientific interest, was of no practical use, but it led to
other inventions, of which it is the base, that are transcendently
useful in every-day life. The transmission of harmonic sounds by
electricity underlies the principle of the telephone. There is a vast
difference, in principle, between the transmission of simple melody,
which is a combination of musical tones transmitted successively--one
tone following another--and the transmission of harmony, which involves
the transmission of two or more tones simultaneously. The former can be
transmitted by a make-and-break current. In the latter case one tone
has to be superposed upon another and must be transmitted with a varying
but a continuously closed current. I make a distinction between a closed
circuit and a closed current. In the case of the arc-light the circuit
is open (that is, broken), technically speaking, but the current is
still flowing. The reason why the Reiss and other metallic contact
telephone transmitters cannot successfully be used for telephone
purposes is that metal points will not allow of sufficient separation of
the transmitting points without breaking the current as well as the
circuit. Carbon contacts admit of a much wider separation without
actually stopping the flow of the current, which latter is a necessity
for perfect telephonic transmission, and it was the use of carbon that
made that form of transmitter a success.
There are other forms, or at least one other form that does not depend
upon the length of the voltaic arc formed when the electrodes are
separated. Of this we will speak another time. Now let us go back to the
domestic incident referred to above.
One evening in the winter of 1873-4 I came home from my laboratory work
and went into the bathroom to make my toilet for dinner. I found my
nephew, Mr. Charles S. Sheppard, together with some of his playmates,
taking electrical "shocks" from a little medical induction-coil that I
heard humming in the closet. He had one terminal of the coil connected
to the zinc lining of the bathtub--which was dry at that time--while he
held the other in his left hand, and with his right was taking shocks
from the lining of the tub by rubbing his hand against the zinc. I
noticed that each time he made contact with the tub, as he rubbed it for
a short distance, a peculiar sound was emitted from under his hand, not
unlike the sound made by the electrotome that was vibrating in the
closet. My interest was immediately aroused, and I took the electrode
out of his hand and for some time experimented with it, going to the
cupboard from time to time to change the rate of vibration of the
electrotome, and thus change the quality of the sound. I noticed that
the sound or tone under my hand, if it could be so called, changed with
each change of the rate of vibration. The thing that most interested me
was that the peculiar characteristics of the noise were reproduced. In
those few minutes I laid out work enough for years of experiment, and as
a result I was late to dinner.
This discovery opened up to my mind the possibility of three things--the
transmission of music and of speech or articulate words through a
telegraph-wire, and the transmission of a number of messages over a
single wire. I constructed a keyboard consisting of one octave and made
a set of reeds tuned to the notes of the scale, and then when some one
would play a melody I could reproduce it in two ways: One by placing my
body in the circuit and rubbing a metal plate--it might be the bottom of
a tin pan, a joint of stovepipe or otherwise--anything that was metal
and would vibrate would give the effect. Another way was to connect an
electromagnet (having a diaphragm or reed across its poles) in the
circuit at the receiving-end and mount it on some kind of a soundboard.
I made a great number of different kinds of receivers that were capable
of receiving either musical or articulate sounds, as has many times been
proven by experiment. I carried two sets of experiments along together;
the one looking toward a system of multiple telegraphy and the other the
transmission of articulate speech. Let us first look into the multiple
telegraph and take the other up under the head of the telephone.
When the electrical keyboard was completed I found that I could transmit
not only a melody but a harmony; that more than one tone could be
transmitted simultaneously. This discovery opened up a long series of
experiments with the view of sending a number of messages simultaneously
by means of musical tones differing in pitch. I had already demonstrated
that several tones could be transmitted at once, but they would speak
all alike (with the same loudness) on the receiving-instrument. I now
went to work on an instrument that responded for one note only and
succeeded beyond my expectations. I made three different kinds of
receiving-instruments. The first was a steel strap about eight inches
long by three-eighths wide. This strap was mounted in an iron frame in
front of an electromagnet. A thumbscrew enabled me to stretch the strap
till it would vibrate at the required pitch. If, for instance, the
sending-reed vibrated at the rate of 100 times per second and the strap
of the receiver was stretched to a tension that would give 100
vibrations per second when plucked, it would then respond to the
vibrations of the sending-reed but not to those of another reed of a
different rate of vibration. If we take mounted tuning-forks tuned in
pairs of different pitches, say four pairs, so that each fork has a mate
that is in exact accord with it, and place them all in the same room,
and sound one of them for a few seconds and then stop it, upon examining
the other forks you will find all of them quiet except the mate of the
one that was sounded. This one will be sounding. If we now sound four of
the forks and then stop them the other four will be sounding from
sympathy because the mate of each one of them has been sounded. If only
two forks differing in pitch are sounded only two of the others will
sound in sympathy. In the first case only one set of sound-waves were
set up in the air, and the fork that found itself in accord with this
set responded. When four forks differing in pitch were sounded there
were four sets of tone-waves superposed upon each other existing in the
air, so that each of the remaining forks found a set of waves in
sympathy with its own natural rate of vibration and so responded.
Now apply this principle to the harmonic telegraph and you can
understand its operation. At the transmitting-end of a line of
wire there are a certain number of forks or reeds kept vibrating
continuously. These reeds each have a fixed rate of vibration
and bear a harmonic relation to each other so as not to have
sound-interference or "beats." At the receiving-end of the line
there are as many electromagnets as there are transmitting-reeds, and
each magnet has a reed or strap in front of it tuned to some one of the
transmitting-reeds, so that each transmitting-reed has a mate in exact
harmony with it at the receiving-end of the line. Keys are so arranged
at the transmitting-end as to throw the tones corresponding to them to
line when depressed. In other words, when the key belonging to battery B
and vibrator 1 is depressed (see Fig. 4) the effect is to send
electrical pulsations through the line corresponding in rate per second
to that of the vibrator. The same is true of battery B' and vibrator 2.
During the time any key is depressed--we will say of tone No. 1--this
tone will be transmitted through the line and be reproduced by its
mate--the one tuned in accord with it--at the receiving-station. By a
succession of long and short tones representing the Morse code a message
can be sent. Numbers two, three and four might be sending at the same
time, but they would not interfere with number one or with each other.
In 1876-7 the writer succeeded in sending eight simultaneous messages
between New York and Philadelphia by the harmonic method.
[Illustration: Fig. 4.
In this diagram, 1 and 2 are tuned reeds; 1A 2A are receivers
tuned to the reeds 1 and 2 respectively; 1 and 1A are in unison,
also 2 and 2A, but the two groups (the 1s and the 2s) differ
from each other in pitch.]
There were two ways of reading by the harmonic method. One was by the
long and short tone-sounds and the other by the ordinary sounder.
The vibration of the receiving-reed was made to open and close a local
circuit like a common Morse relay and thus operate the sounder. It is
useless to try to send a message if the sender and receiver are out of
tune with each other in this system.
What is true in science is true in life. If we are out of tune with our
surroundings we only beat the air, and our efforts are in vain. We get
no sympathetic response.