SOME TELEGRAPHIC AND TELEPHONIC INVENTIONS
In telegraphy messages not only may be received, but also recorded,
by the Morse printer or one of its modifications, but in ordinary
telephony there is no mechanical method of recording messages. This
means that we can communicate by telephone only when we can call up
somebody to receive the message at the other end, and if no one happens
to be within hearing of the telephone bell we are quite helpless. This
is always annoying, and if the message is urgent the delay may be
serious. Several arrangements for overcoming this difficulty by means
of automatic recording mechanism have been invented, but the only
really successful one is the telegraphone.
This instrument is the invention of Waldemar Poulsen, whose apparatus
for wireless telegraphy we shall speak of in the next chapter. The
telegraphone performs at the same time the work of a telephone and
of a phonograph. In the ordinary type of phonograph the record is
made in the form of depressions or indentations on the surface of a
cylinder of wax; these indentations being produced by a stylus actuated
by vibrations set up in a diaphragm by the act of speaking. In the
telegraphone the same result is obtained entirely by electro-magnetic
action. The wax cylinder of the phonograph is replaced by a steel wire
or ribbon, and the recording stylus by an electro-magnet. The steel
ribbon is arranged to travel along over two cylinders or reels kept in
constant rotation, and a small electro-magnet is fixed midway between
the cylinders so that the ribbon passes close above it. This magnet
is connected to the telephone line, so that its magnetism fluctuates
in accordance with the variations in the current in the line. We have
seen that steel retains magnetism imparted to it. In passing over the
electro-magnet the steel ribbon is magnetized in constantly varying
degrees, corresponding exactly with the variations in the line current
set up by the speaker’s voice, and these magnetic impressions are
retained by the ribbon. When the speaker has finished, the telephone
line is disconnected, the ribbon is carried back to the point at which
it started, and the apparatus is connected to the telephone receiver.
The ribbon now moves forward again, and this time it acts like the
speaker’s voice, the varying intensity of its magnetic record producing
corresponding variations in the strength of the magnet, so causing the
receiver diaphragm to reproduce the sounds in the ordinary way.
The magnetic record made in this manner is fairly permanent, and if
desired it may be reproduced over and over again. In most cases,
however, a permanent record is of no value, and so the magnetic
impressions are obliterated in order that the ribbon may be used to
take a new record. This can be done by passing a permanent magnet along
the ribbon, but it is more convenient to have an automatic obliterating
arrangement. This consists of another electro-magnet fixed close to the
recording magnet, so that the ribbon passes over it before reaching the
latter. The obliterating magnet is connected with a battery, and its
unvarying magnetism destroys all traces of the previous record, and
the ribbon passes forward to the recording magnet ready to receive new
impressions.
For recording telephone messages the telegraphone is attached to the
telephone instrument, and by automatically operated switches it is
set working by a distant speaker. It records all messages received
during the absence of its owner, who, on his return, connects it to
his receiver, and thus hears a faithful reproduction of every word.
By speaking into his instrument before going out, the owner can leave
a message stating the time at which he expects to return, and this
message will be repeated by the telegraphone to anybody ringing up
in the meantime. The most recent forms of telegraphone are capable
of recording speeches over an hour in length, and their reproduction
is as clear as that of any phonograph, indeed in many respects it is
considerably more perfect.
Another electrical apparatus for recording speech may be mentioned.
This rejoices in the uncouth name of the Photographophone, and it is
the invention of Ernst Ruhmer, a German. Its working is based upon
the fact that the intensity of the light of the electric arc may be
varied by sound vibrations, each variation in the latter producing a
corresponding variation in the amount of light. In the photographophone
the light of an arc lamp is passed through a lens which focuses it
upon a moving photographic film. By speaking or singing, the light is
made to vary in brilliance, and proportionate effects are produced in
the silver bromide of the film. On developing the film a permanent
record of the changes in the light intensity is obtained, in the
form of shadings of different degrees of darkness. The film is now
moved forward from end to end in front of a fairly powerful lamp. The
light passes through the film, and falls upon a sort of plate made of
selenium. This is a non-metallic substance which possesses the curious
property of altering its resistance to an electric current according
to the amount of light falling upon it; the greater the amount of
light, the more current will the selenium allow to pass. The selenium
plate is connected with a telephone receiver and with a battery. As
the film travels along, its varying shadings allow an ever-changing
amount of light to pass through and fall upon the selenium, which
varies its resistance accordingly. The resulting variations in the
current make the receiver diaphragm give out a series of sounds, which
are exact reproductions of the original sounds made by the voice. The
reproduction of speech by the photographophone is quite good, but as a
rule it is not so perfect as with the telegraphone.
About ten years ago a German inventor, Professor A. Korn, brought
out the first really practical method of telegraphing drawings or
photographs. This invention is remarkable not only for what it
accomplishes, but perhaps still more for the ingenuity with which
the many peculiar difficulties of the process are overcome. Like the
photographophone, Korn’s photo-telegraphic apparatus utilizes the power
of selenium to alter its resistance with the amount of light reaching
it.
Almost everybody is familiar with the terms “positive” and “negative”
as used in photography. The finished paper print is a positive, with
light and shade in the correct positions; while the glass plate from
which the print is made is a negative, with light and shade reversed.
The lantern slide also is a positive, and it is exactly like the
paper print, except that it has a base of glass instead of paper, so
that it is transparent. Similarly, a positive may be made on a piece
of celluloid, and this, besides being transparent, is flexible. The
first step in transmitting on the Korn system is to make from the
photograph to be telegraphed a positive of this kind, both transparent
and flexible. This is bent round a glass drum or cylinder, and fixed
so that it cannot possibly move. The cylinder is given a twofold
movement. It is rotated by means of an electric motor, and at the same
time it is made to travel slowly along in the direction of its length.
In fact its movement is very similar to that of a screw, which turns
round and moves forward at the same time. A powerful beam of light is
concentrated upon the positive. This beam remains stationary, but owing
to the dual movement of the cylinder it passes over every part of the
positive, following a spiral path. Exactly the same effect would be
produced by keeping the cylinder still and moving the beam spirally
round it, but this arrangement would be more difficult to manipulate.
The forward movement of the cylinder is extremely small, so that the
spiral is as fine as it is possible to get it without having adjacent
lines actually touching. The light passes through the positive into the
cylinder, and is reflected towards a selenium cell; and as the positive
has an almost infinite number of gradations of tone, or degrees
of light and shade, the amount of light reaching the cell varies
constantly all the time. The selenium therefore alters its resistance,
and allows a constantly varying current to pass through it, and so to
the transmission line.
At the receiving end is another cylinder having the same rotating and
forward movement, and round this is fixed a sensitive photographic
film. This film is protected by a screen having a small opening, and
no light can reach it except through this aperture. The incoming
current is made to control a beam of light focused to fall upon
the screen aperture, the amount of light varying according to the
amount of current. In this way the beam of light, like the one at the
transmitting end, traces a spiral from end to end of the film, and
on developing the film a reproduction of the original photograph is
obtained. The telegraphed photograph is thus made up of an enormous
number of lines side by side, but these are so close to one another
that they are scarcely noticed, and the effect is something like that
of a rather coarse-grained ordinary photograph.
It is obvious that the success of this method depends upon the
maintaining of absolute uniformity in the motion of the two cylinders,
and this is managed in a very ingenious way. It will be remembered
that one method of securing uniformity in a number of sub-clocks under
the control of a master-clock is that of adjusting the sub-clocks to
go a little faster than the master-clock. Then, when the sub-clocks
reach the hour, they are held back by electro-magnetic action until the
master-clock arrives at the hour, when all proceed together.
A similar method is employed for the cylinders. They are driven by
electric motors, and the motor at the receiving end is adjusted so
as to run very slightly faster than the motor at the sending end.
The result is that the receiving cylinder completes one revolution
a minute fraction of a second before the transmitting cylinder. It
is then automatically held back until the sending cylinder completes
its revolution, and then both commence the next revolution exactly
together. The pause made by the receiving cylinder is of extremely
short duration, but in order that there shall be no break in the spiral
traced by light upon the film, the pause takes place at the point where
the ends of the film come together. In actual practice certain other
details of adjustment are required to ensure precision in working, but
the main features of the process are as described.
Although the above photo-telegraphic process is very satisfactory in
working, it has been superseded to some extent by another process of
a quite different nature. By copying the original photograph through
a glass screen covered with a multitude of very fine parallel lines,
a half-tone reproduction is made. This is formed of an immense number
of light and dark lines of varying breadth, and it is printed in
non-conducting ink on lead-foil, so that while the dark lines are bare
foil, the light ones are covered with the ink. This half-tone is placed
round a metal cylinder having the same movement as the cylinders in the
previous processes, and a metal point, or “stylus” as it is called,
is made to rest lightly upon the foil picture, so that it travels all
over it, from one end to the other. An electrical circuit is arranged
so that when the stylus touches a piece of the bare foil a current is
sent out along the line wire. This current is therefore intermittent,
being interrupted each time the stylus passes over a part of the
half-tone picture covered with the non-conducting ink, the succeeding
periods of current and no current varying with the breadth of the
conducting and the non-conducting lines. This intermittent current goes
to a similar arrangement of stylus and cylinder at the receiving end,
this cylinder having round it a sheet of paper coated with a chemical
preparation. The coating is white all over to begin with, but it turns
black wherever the current passes through it. The final result is that
the intermittent current builds up a reproduction in black-and-white of
the original photograph. In this process also the cylinders have to be
“synchronized,” or adjusted to run at the same speed. Both this process
and the foregoing one have been used successfully for the transmission
of press photographs, notably by the _Daily Mirror_.
Professor Korn has carried out some interesting and fairly successful
experiments in wireless transmission of photographs, but as yet the
wireless results are considerably inferior to those obtained with
a line conductor. For transmitting black-and-white pictures, line
drawings, or autographs by wireless, a combination of the two methods
just mentioned is employed; the second method being used for sending,
and the first or selenium method for receiving. For true half-tone
pictures the selenium method is used at each end.