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FROM the year 1848, when a Scotchman,
Alexander Bain, first devised a scheme for
rapid telegraphy by automatic methods, down to
the beginning of the seventies, many other
inventors had also applied themselves to the
solution of this difficult problem, with only
indifferent success. "Cheap telegraphy" being
the slogan of the time, Edison became arduously
interested in the subject, and at the end of
three years of hard work produced an entirely
successful system, a public test of which was
made on December 11, 1873 when about
twelve thousand (12,000) words were
transmitted over a single wire from Washington
to New York. in twenty-two and one-half
minutes. Edison's system was commercially
exploited for several years by the Automatic
Telegraph Company, as related in the preceding
narrative.
As a premise to an explanation of the principles
involved it should be noted that the transmission
of telegraph messages by hand at a rate of fifty
words per minute is considered a good average
speed; hence, the availability of a telegraph
line, as thus operated, is limited to this
capacity except as it may be multiplied by two
with the use of the duplex, or by four, with
the quadruplex. Increased rapidity of
transmission may, however, be accomplished by
automatic methods, by means of which, through
the employment of suitable devices, messages may
be stamped in or upon a paper tape, transmitted
through automatically acting instruments, and be
received at distant points in visible
characters, upon a similar tape, at a rate
twenty or more times greater--a speed far
beyond the possibilities of the human hand to
transmit or the ear to receive.
In Edison's system of automatic telegraphy a
paper tape was perforated with a series of round
holes, so arranged and spaced as to represent
Morse characters, forming the words of the
message to be transmitted. This was done in a
special machine of Edison's invention, called
a perforator, consisting of a series of punches
operated by a bank of keys--typewriter
fashion. The paper tape passed over a
cylinder, and was kept in regular motion so as
to receive the perforations in proper sequence.
The perforated tape was then placed in the
transmitting instrument, the essential parts of
which were a metallic drum and a projecting arm
carrying two small wheels, which, by means of a
spring, were maintained in constant pressure on
the drum. The wheels and drum were electrically
connected in the line over which the message was
to be sent. current being supplied by batteries
in the ordinary manner.
When the transmitting instrument was in
operation, the perforated tape was passed over
the drum in continuous, progressive motion.
Thus, the paper passed between the drum and the
two small wheels, and, as dry paper is a non-
conductor, current was prevented from passing
until a perforation was reached. As the paper
passed along, the wheels dropped into the
perforations, making momentary contacts with the
drum beneath and causing momentary impulses of
current to be transmitted over the line in the
same way that they would be produced by the
manipulation of the telegraph key, but with much
greater rapidity. The perforations being so
arranged as to regulate the length of the
contact, the result would be the transmission of
long and short impulses corresponding with the
dots and dashes of the Morse alphabet.
The receiving instrument at the other end of the
line was constructed upon much the same general
lines as the transmitter, consisting of a
metallic drum and reels for the paper tape.
Instead of the two small contact wheels,
however, a projecting arm carried an iron pin or
stylus, so arranged that its point would
normally impinge upon the periphery of the drum.
The iron pin and the drum were respectively
connected so as to be in circuit with the
transmission line and batteries. As the
principle involved in the receiving operation was
electrochemical decomposition, the paper tape
upon which the incoming message was to be
received was moistened with a chemical solution
readily decom- posable by the electric current.
This paper, while still in a damp condition,
was passed between the drum and stylus in
continuous, progressive motion. When an
electrical impulse came over the line from the
transmitting end, current passed through the
moistened paper from the iron pin, causing
chemical decomposition, by reason of which the
iron would be attacked and would mark a line on
the paper. Such a line would be long or short,
according to the duration of the electric
impulse. Inasmuch as a succession of such
impulses coming over the line owed their origin
to the perforations in the transmitting tape, it
followed that the resulting marks upon the
receiving tape would correspond thereto in their
respective lengths. Hence, the transmitted
message was received on the tape in visible dots
and dashes representing characters of the Morse
alphabet.
The system will, perhaps, be better understood
by reference to the following diagrammatic sketch
of its general principles:
Some idea of the rapidity of automatic
telegraphy may be obtained when we consider the
fact that with the use of Edison's system in
the early seventies it was common practice to
transmit and receive from three to four thousand
words a minute over a single line between New
York and Philadelphia. This system was
exploited through the use of a moderately paid
clerical force.
In practice, there was employed such a number
of perforating machines as the exigencies of
business demanded. Each machine was operated by
a clerk, who translated the message into
telegraphic characters and prepared the
transmitting tape by punching the necessary
perforations therein. An expert clerk could
perforate such a tape at the rate of fifty to
sixty words per minute. At the receiving end
the tape was taken by other clerks who translated
the Morse characters into ordinary words, which
were written on message blanks for delivery to
persons for whom the messages were intended.
This latter operation--"copying." as it was
called--was not consistent with truly
economical business practice. Edison therefore
undertook the task of devising an improved system
whereby the message when received would not
require translation and rewriting, but would
automatically appear on the tape in plain letters
and words, ready for instant delivery.
The result was his automatic Roman letter
system, the basis for which included the
above-named general principles of perforated
transmission tape and electrochemical
decomposition. Instead of punching Morse
characters in the transmission tape however, it
was perforated with a series of small round holes
forming Roman letters. The verticals of these
letters were originally five holes high. The
transmitting instrument had five small wheels or
rollers, instead of two, for making contacts
through the perforations and causing short
electric impulses to pass over the lines. At
first five lines were used to carry these
impulses to the receiving instrument, where
there were five iron pins impinging on the drum.
By means of these pins the chemically prepared
tape was marked with dots corresponding to the
impulses as received, leaving upon it a legible
record of the letters and words transmitted.
For purposes of economy in investment and
maintenance, Edison devised subsequently a plan
by which the number of conducting lines was
reduced to two, instead of five. The verticals
of the letters were perforated only four holes
high, and the four rollers were arranged in
pairs, one pair being slightly in advance of the
other. There were, of course, only four pins
at the receiving instrument. Two were of iron
and two of tellurium, it being the gist of
Edison's plan to effect the marking of the
chemical paper by one metal with a positive
current, and by the other metal with a negative
current. In the following diagram, which shows
the theory of this arrangement, it will be seen
that both the transmitting rollers and the
receiving pins are arranged in pairs, one pair
in each case being slightly in advance of the
other. Of these receiving pins, one pair--1
and 3--are of iron, and the other pair--2
and 4--of tellurium. Pins 1-2 and 3-4
are electrically connected together in other
pairs, and then each of these pairs is connected
with one of the main lines that run respectively
to the middle of two groups of batteries at the
transmitting end. The terminals of these groups
of batteries are connected respectively to the
four rollers which impinge upon the transmitting
drum, the negatives being connected to 5 and
7, and the positives to 6 and 8, as denoted
by the letters N and P. The transmitting and
receiving drums are respectively connected to
earth.
In operation the perforated tape is placed on
the transmission drum, and the chemically
prepared tape on the receiving drum. As the
perforated tape passes over the transmission drum
the advanced rollers 6 or 8 first close the
circuit through the perforations, and a positive
current passes from the batteries through the
drum and down to the ground; thence through the
earth at the receiving end up to the other drum
and back to the batteries via the tellurium pins
2 or 4 and the line wire. With this positive
current the tellurium pins make marks upon the
paper tape, but the iron pins make no mark. In
the merest fraction of a second, as the
perforated paper continues to pass over the
transmission drum, the rollers 5 or 7 close
the circuit through other perforations and t e
current passes in the opposite direction, over
the line wire, through pins 1 or 3, and
returns through the earth. In this case the
iron pins mark the paper tape, but the tellurium
pins make no mark. It will be obvious,
therefore, that as the rollers are set so as to
allow of currents of opposite polarity to be
alternately and rapidly sent by means of the
perforations, the marks upon the tape at the
receiving station will occupy their proper
relative positions, and the aggregate result
will be letters corresponding to those perforated
in the transmission tape.
Edison subsequently made still further
improvements in this direction, by which he
reduced the number of conducting wires to one,
but the principles involved were analogous to the
one just described.
This Roman letter system was in use for several
years on lines between New York,
Philadelphia, and Washington, and was so
efficient that a speed of three thousand words a
minute was attained on the line between the two
first-named cities.
Inasmuch as there were several proposed systems
of rapid automatic telegraphy in existence at the
time Edison entered the field, but none of them
in practical commercial use, it becomes a matter
of interest to inquire wherein they were
deficient, and what constituted the elements of
Edison's success.
The chief difficulties in the transmission of
Morse characters had been two in number, the
most serious of which was that on the receiving
tape the characters would be prolonged and run
into one another, forming a draggled line and
thus rendering the message unintelligible. This
arose from the fact that, on account of the
rapid succession of the electric impulses, there
was not sufficient time between them for the
electric action to cease entirely. Consequently
the line could not clear itself, and became
surcharged, as it were; the effect being an
attenuated prolongation of each impulse as
manifested in a weaker continuation of the mark
on the tape, thus making the whole message
indistinct. These secondary marks were called
"tailings."
For many years electricians had tried in vain to
overcome this difficulty. Edison devoted a
great deal of thought and energy to the
question, in the course of which he experimented
through one hundred and twenty consecutive
nights, in the year 1873, on the line
between New York and Washington. His
solution of the problem was simple but
effectual. It involved the principle of
inductive compensation. In a shunt circuit with
the receiving instrument he introduced
electromagnets. The pulsations of current
passed through the helices of these magnets,
producing an augmented marking effect upon the
receiving tape, but upon the breaking of the
current, the magnet, in discharging itself of
the induced magnetism, would set up momentarily
a counter-current of opposite polarity. This
neutralized the "tailing" effect by clearing
the line between pulsations, thus allowing the
telegraphic characters to be clearly and
distinctly outlined upon the tape. Further
elaboration of this method was made later by the
addition of rheostats, condensers, and local
opposition batteries on long lines.
The other difficulty above referred to was one
that had also occupied considerable thought and
attention of many workers in the field, and
related to the perforating of the dash in the
transmission tape. It involved mechanical
complications that seemed to be insurmountable,
and up to the time Edison invented his
perforating machine no really good method was
available. He abandoned the attempt to cut
dashes as such, in the paper tape, but instead
punched three round holes so arranged as to form
a triangle. A concrete example is presented in
the illustration below, which shows a piece of
tape with perforations representing the word
"same."
The philosophy of this will be at once perceived
when it is remembered that the two little wheels
running upon the drum of the transmitting
instrument were situated side by side,
corresponding in distance to the two rows of
holes. When a triangle of three holes,
intended to form the dash, reached the wheels,
one of them dropped into a lower hole. Before
it could get out, the other wheel dropped into
the hole at the apex of the triangle, thus
continuing the connection, which was still
further prolonged by the first wheel dropping
into the third hole. Thus, an extended contact
was made, which, by transmitting a long
impulse, resulted in the marking of a dash upon
the receiving tape.
This method was in successful commercial use for
some time in the early seventies, giving a speed
of from three to four thousand words a minute
over a single line, but later on was superseded
by Edison's Roman letter system, above
referred to.
The subject of automatic telegraphy received a
vast amount of attention from inventors at the
time it was in vogue. None was more earnest or
indefatigable than Edison, who, during the
progress of his investigations, took out
thirty-eight patents on various inventions
relating thereto, some of them covering chemical
solutions for the receiving paper. This of
itself was a subject of much importance and a
vast amount of research and labor was expended
upon it. In the laboratory note-books there
are recorded thousands of experiments showing
that Edison's investigations not only included
an enormous number of chemical salts and
compounds, but also an exhaustive variety of
plants, flowers, roots, herbs, and barks.
It seems inexplicable at first view that a
system of telegraphy sufficiently rapid and
economical to be practically available for
important business correspondence should have
fallen into disuse. This, however, is made
clear--so far as concerns Edison's invention
at any rate--in Chapter VIII of the
preceding narrative.
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