The most recent of the brilliant series of telegraphic marvels which
has from time to time, and especially of late, engaged the attention
of the world, is the "telegraphic pen" of Mr. E. A. Cowper, the well
known engineer of Great George street, Westminster. This ingenious
apparatus, which constitutes the first real telegraph, was publicly
shown by its inventor at the meeting of the Society of Telegraph
Engineers on Wednesday, February 26.
There had been no lack of copying telegraphs hitherto. We have
Bakewell's, Casselli's, Meyer's, and D'Arlincourt's, so recently tried
at our General Post Office by Mr. Preece. All of these instruments
telegraph an almost perfect copy of the writing or sketch submitted to
them by means of synchronous mechanism. But the process is necessarily
complex and slow; whereas by the new device a person may take the
writing pencil in his hand, and himself transmit his message in the
act of writing it.
The principle which guided Mr. Cowper to a solution of the problem
which he has successfully overcome, is the well known mathematical
fact that the position of any point in a curve can be determined by
its distance from two rectangular co-ordinates. It follows, then, that
every position of the point of a pencil, stylus, or pen, as it forms a
letter, can be determined by its distance from two fixed lines, say
the adjacent edges of the paper. Moreover it is obvious that if these
distances could be transmitted by telegraph and recombined so as to
give a resultant motion to a duplicate pen, a duplicate copy of the
original writing would be produced. But inasmuch as the writing stylus
moves continuously over the paper, the process of transmission would
require to be a continuous one; that is to say, the current traversing
the telegraph line, and conveying the distances in question (or what
comes to the same thing, the up and down, and direct sidelong ranges
of the stylus) would require to vary continuously in accordance with
the range to be transmitted.
Mr. Cowper effects this by employing two separate telegraphic
circuits, each with its own wire, battery, sending, and receiving
apparatus. One of these circuits is made to transmit the up and down
component writing of the pencil's motion, while the other
simultaneously transmits its sidelong component. At the receiving
station these two components are then recomposed by a pantograph
arrangement of taut cords, or levers, and the resultant motion is
communicated to the duplicate pen at that place. The plan adopted by
Mr. Cowper to transmit each continuously varying component is to cause
the resistance of the circuit to vary very closely with the component
in question. Fig. 5 shows how the apparatus is theoretically arranged
for this purpose. P is the writing style, which is held in the
writer's hand in the ordinary way, while he shapes the letters one by
one on paper pulled uniformly underneath by means of clockwork. To P
are attached, at right angles, two arms, a a, one for each circuit;
but as it is only necessary to consider one of the circuits, say that
sending up and down motions, we will confine our attention for the
present to the arm, a. One pole of the sending battery, B, is
connected to the arm, a, the other pole being connected to earth. Now
the arm, a, is fitted with a sliding contact at its free extremity,
and as the pencil, P, is moved in writing, a slides lengthwise across
the edges of a series of thin metal contact plates, C, insulated from
each other by paraffined paper. Between each pair of these plates
there is a resistance coil, C, and the last of these is connected
through the last plate to the line, L. It will be seen that as a
slides outward across the plates the current from the battery has to
pass through fewer coils, since a short-circuits a number of coils
proportional to its motion. But the fewer of these coils in circuit
the stronger will be the current in the line; so that the extent of
the motion of the arm, a, in the direction of its length, that is to
say, the direct component of the motion of the pencil along the line
of the arm, a, is attended by a corresponding change in the current
traversing the line. If the pencil makes a long up and down stroke
there will be a strong current in the line, if a short one there will
be a weak current, and so on. A precisely similar arrangement is used
to transmit the sidelong motion of the pencil along the line, L.
[Illustration: Fig. 5.]
The current from the line, L, flows at the receiving station through a
powerful galvanometer, G, to earth. The galvanometer has a stout
needle, one tip of which is connected to a duplicate pen, P, by a
thread, t, which is kept taut by a second thread stretched by a
spring, s'. The current from the line, L', flows through a similar
galvanometer, G', to earth. The needle of G' is also connected to the
pen, P, by a taut thread, t', stretched by means of the spring, s.
Now, since the needle of each of these galvanometers deflects in
proportion to the strength of the current flowing through its coil,
the points of these two needles keep moving with the varying currents.
But since these currents vary the motions of the sending pen, the
receiving pen controlled by the united movements of the needles will
trace out a close copy of the original writing. We give on another
page a facsimile of a sentence written by Mr. Cowper's telegraph.
[Illustration: THE COWPER WRITING TELEGRAPH.]
The receiving pen is a fine glass siphon, drawing off aniline ink from
a small glass holder. There are thirty-two coils, C, in each circuit,
with a corresponding number of contact plates, c, so as to get
accuracy of working. A few Daniell's cells are sufficient to operate
the apparatus, and writing has been already sent successfully over a
line 40 miles in length. The writing may be received either of the
same size or larger or smaller than the original, as the case may be.
At present the writing must not be too hurried, that is, unless the
characters are bold and well formed; but further improvement will, of
course, quicken the working of the apparatus.
The engravings, Figs. 1 to 4, illustrate the actual apparatus. Fig. 4
is a plan of the sending instrument, with the writing pencil, a, the
traveling paper, b, the light connecting rods or arms, d (which
correspond to a in the theoretical diagram above), the series of metal
contact plates over which these arms slide, the resistance coils
connected to these plates, and the battery and line wires. It will be
seen that each arm, d, is connected to its particular battery, and
each set of contact plates to its particular line. Fig. 3 is an
elevation of the sending instrument, in which a is the pencil as
before, c c the contact plates over which the arms, d d, slide, f f
the coils, and b the traveling slip of paper.
Fig. 2 is a plan of the receiving instrument, in which h h are the
light pivoted needles surrounded by coils of fine insulated copper
wires, i i, and controlled in their zero position by the
electro-magnets, j j j j, placed underneath, the whole forming a pair
of galvanoscopes or current detecters, one for each line. It will be
understood that the varying currents from the lines are allowed to
flow through the coils, i i, so as to deflect the needles, and that
the deflections of the needles follow, so to speak, the variations of
the currents. The electro-magnets are magnetized by a local battery;
permanent magnets might, however, take their place with a gain in
simplicity.
Now the writing pen, k, is connected to the nearest tip of the needle,
h, of each galvanoscope by threads, n n, which are kept taut by the
fibers, o_{1} o_{2} o_{3}, the springs, o, and the pins, o_{4}. In
this way the motions of the needles are recombined in the motion of
the duplicate pen upon the paper, p.
Fig. 1 is an elevation of the receiving instrument, in which i i are
the coils as before, j j j j the controlling electro-magnets, k is the
writing siphon dipping with its short leg into the ink well, m, and l
is the bridge from which the writing siphon is suspended by means of a
thread and spring. The long leg of the siphon reaches down to the
surface of the paper, p, which is pulled along beneath it in contact
with the film of ink filling the point of the tube. When the siphon is
at rest its point marks a zero line along the middle of the paper, but
when the receiver is working, the siphon point forms each letter of
the message upon the paper as it passes.--_Engineering._
* * * * *