By THEO. F. TAYLOR.
Since 1838 much has been done toward increasing the carrying capacity of
a single wire. In response to your invitation I will relate my
experience upon the Postal's large coppered wire, in an effort to
transmit 800 words per minute over a 1,000 mile circuit, and add my mite
to the vast sum of knowledge already possessed by electricians.
As an introduction, I shall mention a few historical facts, but do not
propose to write in this article even a short account of the different
automatic systems, and I must assume that my readers are familiar with
modern automatic machines and appliances.
In 1870, upon the completion of the Automatic Company's 7 ohm wire
between New York and Washington, it happened that Prof. Moses G. Farmer
was in the Washington office when the first message was about to be
sent, and upon being requested, he turned the "crank" and transmitted
the message to New York, at the rate of 217 words per minute.
Upon his return to New York he co-operated with Mr. Prescott in
experiments on W.U. wires, their object being to determine what could be
done on iron wires with the Bain system. A good No. 8 wire running from
New York to Boston was selected, reinsulated, well trimmed, and put in
first-class electrical condition, previous to the test. The "Little"
chemical paper was used.
The maximum speed attained on this wire was 65 words per minute.
About the same time George H. Grace used an electro magnet on the
automatic line with such good effect that the speed on the New
York-Washington circuit was increased to 450 words per minute.
Then a platina stylus or pen was substituted for the iron pen in
connection with iodide paper, and the speed increased to 900 words per
minute.
In 1880, upon the completion of the Rapid Company's 6 ohm wire, between
New York and Boston, 1,200 words per minute were transmitted between the
cities above named.
In 1882, I was employed by the Postal Telegraph Company to put the Leggo
automatic system into practical shape, and, if possible, transmit 800
words per minute between New York and Chicago.
It was proposed to string a steel-copper wire, the copper on which was
to weigh 500 lb. to the mile.
When complete, the wire was rather larger than No. 3, English gauge, but
varied in diameter, some being as large as No. 1, and it averaged 525
lb. of copper per mile and = 1.5 ohms. The surface of this wire was,
however, large.
Dr. Muirhead estimated its static capacity at about 10 M.F., which
subsequent tests proved to be nearly correct.
It will be understood that this static capacity stood in the way of fast
transmission.
Resistance and static capacity are the two factors that determine speed
of signaling.
The duration of the variable state is in proportion to the square of the
length of the conductor, so that the difficulties increase very greatly
as the wire is extended beyond ordinary limits. According to Prescott,
"The duration of the variable condition in a wire of 500 miles is
250,000 times as long as in a wire of 1 mile."
In other words, a long line _retains a charge_, and time must be allowed
for at least a falling off of the charge to a point indicated by the
receiving instrument as zero.
In the construction of the line care was taken to insure the _lowest
possible resistance_ through the circuit, even to the furnishing of the
river cables with conductors weighing 500 lb. per mile.
Ground wires were placed on every tenth pole.
When the first 100 miles of wire had been strung, I was much encouraged
to find that we could telegraph without any difficulty past the average
provincial "ground," provided the terminal grounds were good.
When the western end of this remarkable wire reached Olean, N.Y., 400
miles from New York, my assistant, Mr. S.K. Dingle, proceeded to that
town with a receiving instrument, and we made the first test.
I found that 800 words, or 20,000 impulses, per minute, could be
transmitted in Morse characters over that circuit _without compensation_
for static.
In other words, the old Bain method was competent to telegraph 800 words
per minute on the 400 miles of 1.5 ohm wire.
The trouble began, however, when the wire reached Cleveland, O., about
700 miles from New York.
Upon making a test at Cleveland, I found the signals made a continuous
black line upon the chemical paper. I then placed both ends of the wire
to earth through 3,000 ohms resistance, and introduced a small auxiliary
battery between the chemical paper and earth.
The auxiliary or opposing battery was placed in the same circuit with
the transmitting battery, and the currents which were transmitted from
the latter through the receiving instrument reached the earth by passing
directly through the opposing battery.
The circuit of the opposing battery was permanently completed,
independently of the transmitting apparatus, through both branch
conductors and artificial resistances.
The auxiliary battery at the receiving station normally maintained upon
the main line a continuous electric current of a negative polarity,
which did not produce a mark upon the chemical paper.
When the transmitting battery was applied thereto, the excessive
electro-motive force of the latter overpowered the current from the
auxiliary battery and exerted, by means of a positive current, an
electro-chemical action upon the chemical receiving paper, producing a
mark.
Immediately upon the interruption of the circuit of the transmitting
battery, the unopposed current from the auxiliary battery at the
receiving station flowed back through the paper and into the main line,
thereby both neutralizing the residual or inductive current, which
tended to flow through the receiving instrument, and serving to clear
the main line from electro-static charge.
The following diagram illustrates my method:
Referring to this diagram, A and B respectively represent a transmitting
and a receiving station of an automatic telegraph. These stations are
united in the usual manner by a main line, L. At the transmitting
station, A, is placed a transmitting battery, E, having its positive
pole connected by a conductor, 2, with the metallic transmitting drum,
T. The negative pole of the battery, E, is connected with the earth at G
by a conductor, 1. A metallic transmitting stylus, t, rests upon the
surface of the drum, T, and any well known or suitable mechanism may be
employed for causing an automatic transmitting pattern slip, P, to pass
between the stylus and the drum. The transmitting or pattern slip, P, is
perforated with groups of apertures of varying lengths and intervals as
required to represent the dispatch which it is desired to transmit, by
an arbitrary system of signs, such, for example, as the Morse
telegraphic code.
At the receiving station, B, is placed a recording apparatus, M, of any
suitable or well known construction. A strip of chemically prepared
paper, N, is caused to pass rapidly and uniformly between the drum, M',
and the stylus, m, of this instrument in a well known manner. The drum,
M', is connected with the earth by conductors, 4 and 3, between which is
placed the auxiliary battery, E, the positive or marking pole of this
battery being connected with the drum and the negative pole with the
earth. The electro-motive force of the battery, E', is preferably made
about one-third as great as that of the battery, E.
[Illustration]
Extending from a point, o, in the main line, near the transmitting
station, to the earth at G, is a branch conductor, l, containing an
adjustable artificial resistance, R. A similar conductor, ll, extends
from a point, o', near the receiving terminal of the line, L, to the
conductor, 3, in which an artificial resistance, R', is also included,
this resistance being preferably approximately equal to the resistance,
R. The proportions of the resistance of the main line and the artificial
resistances which I prefer to employ may be approximately indicated as
follows: Assuming the resistance of the main line to be 900 ohms, the
resistance, R, and R', should be each about 3,000 ohms. The main
battery, E, should then comprise about 90 cells, and the auxiliary
battery, E', 30 cells.
The operation of my improved system is as follows: While the apparatus
is at rest a constant current from the battery, E', traverses the line,
L, and the branch conductors, l, and ll, dividing itself between them,
in inverse proportion to their respective resistances, in accordance
with the well-known law of Ohm. When the transmitting pattern strip, P,
is caused to pass between the roller, T, and the stylus, t, electric
impulses will be transmitted upon the line, L, from the positive pole of
the battery, E, which will traverse the main line, L, the two branch
lines, l, and ll, and their included resistances, and also the receiving
instrument, M. The greater portion of this current will, however, on
account of the less resistance offered, traverse the receiving
instrument, M, and the auxilary battery, E'. The current from the
last-named battery will thus be neutralized and overpowered, and the
excess of current from the main battery, E, will act upon the chemically
prepared paper and record in the form of dots and dashes or like
arbitrary characters the impulses which are transmitted.
Immediately on the cessation of each impulse, the auxiliary battery, E',
again acts to send an impulse of positive polarity through the receiving
paper and stylus in the reverse direction and through the line, L, which
returns to the negative pole of the battery by way of the artificial
resistances, R and R'. Such an impulse, following immediately upon the
interruption of the circuit of the transmitting battery, acts to destroy
the effect of the "tailing" or static discharge of the line, L, upon the
receiving instrument, and also to neutralize the same throughout the
line. By thus opposing the discharge of the line by a reverse current
transmitted directly through the chemical paper, a sharply defined
record will in all cases be obtained; and by transmitting the opposing
impulse through the line, the latter will be placed in a condition to
receive the next succeeding impulse and to record the same as a sharply
defined character.
This arrangement was made on the New York-Cleveland circuit, and the
characters were then clearly defined and of uniform distinctness. The
speed of transmission on this circuit was from 1,000 to 2,000 words per
minute.
Upon the completion of the wire to Chicago, total distance 1,050 miles,
including six miles of No. 8 iron wire through the city, the maximum
speed was found to be 1,200 words per minute, and to my surprise the
speed was not affected by the substitution of an underground conductor
for the overhead wire.
The underground conductor was a No. 16 copper wire weighing 67 pounds
per mile, in a Patterson cable laid through an iron pipe.
I used 150 cells of large Fuller battery on the New York-Chicago
circuit, and afterward with 200 cells in first class condition,
transmitted 1,500 words, or 37,000 impulses, per minute from 49
Broadway, New York, to our test office at Thirty-ninth Street, Chicago.
The matter was always carefully counted, and the utmost care taken to
obtain correct figures.
It may be mentioned as a curious fact that we not only send 1,200 words
per minute through 1,050 miles of overhead wire and five miles of
underground cable, but also through a second conductor in No. 2 cable
back to Thirty-ninth Street, and then connected to a third underground
conductor in No. 1 cable back to Chicago main office, in all about
fifteen miles of underground, through which we sent 1,200 words per
minute and had a splendid margin.--_Electrical World_.
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[ELECTRICAL REVIEW].