<h2><SPAN name="CHAPTER_XIV" id="CHAPTER_XIV"></SPAN>CHAPTER XIV.</h2>
<h3>WAY DUPLEX SYSTEM.</h3>
<p>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.</p>
<p>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.<span class="pagenum"><SPAN name="Page_130" id="Page_130"></SPAN></span>
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.</p>
<p>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<span class="pagenum"><SPAN name="Page_131" id="Page_131"></SPAN></span>
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.</p>
<p>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.</p>
<p>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 mov<span class="pagenum"><SPAN name="Page_132" id="Page_132"></SPAN></span>ing
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<span class="pagenum"><SPAN name="Page_133" id="Page_133"></SPAN></span>
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.</p>
<p>Some years ago an experiment of this kind
was made on one of the roads between Milwaukee
and Chicago.</p>
<p>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.</p>
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