WAY DUPLEX SYSTEM.
A novel form of double transmission was invented by the writer soon
after the completion of the harmonic system, and was an outgrowth of it.
It is still in use on some of the railroad-lines. An ordinary railroad
telegraph-line has an instrument in circuit in every office along the
road, chiefly for purposes of train-dispatching. As we have heretofore
explained, whenever any one office is sending, the dispatch is heard in
all of the offices. The "Way duplex" system permits of the use of the
line for through business simultaneously with the operation of the local
offices. That is to say, any station along the line may be telegraphing
with any other station by the ordinary Morse method, and at the same
time messages may be passing back and forth between the two end offices.
This is accomplished by the following method: At each end of the line
there is a tuned reed, such as we have described in our last chapter,
that is kept constantly in vibration by a local battery during working
hours. This vibrator is so arranged in relation to the battery that
whenever the key belonging to it is depressed the current all through
the line is rendered vibratory. There is also in circuit at each end of
the line a harmonic relay, that is tuned in accord with the vibrating
reed of the sender. If either key belonging to this part of the system
is opened, as in the act of sending a message, these harmonic relays,
being tuned in sympathy with the sending-vibrator, will respond, thus
sending Morse characters made up of a tone broken into dots and dashes.
This tone can be read directly from the relay, or, as is usually the
case, it causes the sounder to operate in the common way.
You will at once inquire why the ordinary Morse instruments in the local
offices are not affected by these vibratory signals, and also why the
harmonic instruments at the end office are not affected by the working
of the local offices. The local office does not open the circuit
entirely, but simply cuts out a resistance by the operation of the
special harmonic key. When a resistance is thrown into an electric
circuit it weakens the current in proportion to the amount of resistance
interposed. You will see that there is some current still left in the
line when the key is open, but the spring of the relay at the local
office is so adjusted as to pull the armatures away from the magnets
whenever the current is weakened by throwing in the resistance, so that
by this means an ordinary Morse telegraphic relay may be worked without
ever entirely opening the circuit. In the Way duplex system there is a
resistance at each station that is cut in and out by the operation of
its key, which causes all the instruments in the line to work
simultaneously except the two harmonic relays located one at each end of
the line. These will not respond to anything but the vibratory signal.
In order to prevent the Morse relays at the local offices from
responding to the vibratory current a condenser is connected around
them. This condenser serves two purposes: It enables the short impulses
of the vibrating current to pass around the relays without having to be
resisted by the coils of the magnets, and between the pulsations each
condenser will discharge through the relay at the local offices, and
thus fill in the gap between the pulsations, producing the effect on the
relay of a steady current. When a line is thus equipped it may be
treated in every respect as two separate wires, one of them doing way
business and the other through business. It is a curious blending of
science and mechanism.
Another interesting application was made of the system of transmission
by musical tones--by Edison, some years ago. We refer to the
transmission of messages to and from a moving railroad-train with the
head office at the end of the line. In this case the message was
transmitted a part of the distance through the air;--another instance of
wireless telegraphy. The operation was as follows: One of the wires
strung on the poles nearest to the track was fitted up with a vibrator
and key at the end of the line similar to that of the Way duplex just
described. In one of the cars was another battery, key and vibrator, and
as only one tone was used, no tone-selecting device or harmonic relay
was needed, but instead an ordinary receiving-telephone was used to read
the long and short sounds sent over the lines. One end of the battery in
the car was connected through the wheels to the earth, while the other
end was connected to the metal roof of the car. Being thus equipped, we
will suppose our train to be out on the road forty or fifty miles from
either end of the line, moving at the rate of forty miles an hour. The
operator at Chicago, say, wishes to send a message to the moving train;
he operates his key in the ordinary manner, which makes the current on
the line vibratory during the time the key is depressed. These
electrical vibrations cause magnetic vibrations, or ether-waves, to
radiate in every direction from the wire, at right angles to the
direction of the current, like rays of light. When they strike the roof
of the car they create electrical impulses in the metal by induction
(described in Chap. VI). These impulses pass through a telephone located
in the car to the ground. A Morse operator listening, with the telephone
to his ear, will hear the message through the medium of a musical tone
chopped up into the Morse code. In like manner the operator in the car
may transmit a message to the roof of the car and thence through the air
to the wire, which will be heard, by any one listening, in a telephone
which is connected in that circuit,--and, as a matter of fact, it will
be heard from any wire that may be strung on any of the poles on either
side of the road.
Some years ago an experiment of this kind was made on one of the roads
between Milwaukee and Chicago.
What wonderful things can be done with electricity! As a servant of man
it is reliable and accurate--seeming almost to have the qualities of
docility--when under intelligent direction, that is in accord with the
laws of nature; but under other conditions it changes from the willing
servant to a hard master, hesitating not to destroy life or property
without regard to persons or things.