HOW PICTURES CAN BE SENT BY WIRE
The sending of a message by telegraph is easily understandable. Various
combinations of two simple signs, such as short sounds and long sounds,
can readily be made to indicate letters by which the words can be spelt
out.
Nor does the sending of sound over a wire make a very great demand upon
the credulity. We all know that sound consists of innumerable little
waves in the air, and by the simplest of devices these can be converted
into variations in an electric current, which variations, by means
equally simple, can be made to re-convert themselves back into sound
waves at the other end.
But to transmit a picture is another matter altogether. It seems barely
possible in the case of a drawing such as a pen-and-ink sketch, which
consists of a comparatively small number of definite lines; but with a
shaded sketch or a photograph, with its gradations of light and
shadow--to transmit such would seem to be beyond the bounds of
possibility, did we not know that it has been done. The description of
the methods will therefore constitute a not uninteresting subject for a
chapter.
It is worthy of remark that an attempt along these lines was made many
years ago by a man named Caselli, and a description of this pioneer
apparatus will form a good introduction to the later developments.
In Fig. 13 we see a square which represents a sheet of tinfoil, upon
which is drawn, in non-conductive ink, a simple geometrical figure. The
ink may be grease, or shellac varnish, indeed there are many substances
which are available for use as an insulating ink. Across the square
there are a number of parallel dotted lines, but these, it must be
understood, are not actually drawn upon the foil--their purpose will be
apparent in a moment.
Suppose that we connect the foil to one pole of a battery, and the other
pole by a flexible wire to a metal pen or stylus. If we place the point
of the pen in contact with the foil, we make a complete circuit, through
which, of course, current will flow. But if, with it, we touch one of
the non-conductive lines, there will be no current.
[Illustration: FIG. 13]
[Illustration: FIG. 14]
Taking a ruler, then, let us draw the point of the stylus across the
foil in a series of parallel straight lines. It is these excursions of
the stylus which the dotted lines are intended to represent. For nearly
the whole of the time current will be flowing; but whenever the stylus
is crossing one of the lines of non-conductive ink there will be a
momentary cessation. Thus, the reader will begin to perceive, we obtain
what we may call an electrical representation of the figure drawn upon
the foil.
And now let us turn to Fig. 14. There, too, is a square, but in this
case it is not foil, but paper which has been soaked in prussiate of
potash. The reason for introducing this chemical is that it is
susceptible to electrical action. Wherever current passes through it, it
becomes changed into Prussian blue, so that if we place the point of a
pen upon the paper, and cause current to flow out of that point through
the paper, there we get a blue dot. If, while the current is flowing, we
draw the pen along, we get a blue line.
Fig. 13 therefore represents in principle the sending apparatus of
Caselli's writing telegraph, while Fig. 14 represents the receiving
instrument. The two pens are connected together by the main wire, in
such a manner that, when the point of the one is in contact with the
bare foil current flows out of the other and into the paper; but as the
former crosses an ink line all current ceases.
If, then, while the sending pen is drawn line by line across the foil,
the other is drawn at the same speed, line by line, across the
chemically prepared paper, we shall get on the latter a series of lines
as shown in Fig. 14 almost continuous, but broken here and there. Each
breakage represents a passage of the sending pen across a line, and
taken together, as will be seen, they constitute a reproduction of the
geometrical figure drawn upon the foil. As shown, the lines are rather
far apart, and so the reproduction is not a very good one. They are only
drawn so, however, in order that the principle may be shown the more
clearly. They may be drawn so that they overlap, and then the effect is
very much better, the received picture being almost an exact
reproduction of the other.
It will be noticed that an essential to the success of this method is
that the two pens should move in perfect unison, and that was the great
difficulty. Caselli used an arrangement of pendulums, the best thing
available at the time.
The reproduction is, in photographic language, a negative, a somewhat
unsatisfactory feature of the method. A simple modification, however, of
the electrical connections will reverse that, so that the reproduction
shall be a positive. There are two ways of cutting off a current from
any particular circuit. One is to interpose a resistance, through which
current cannot pass in an appreciable quantity, and the other is to
provide a second path for the current so much easier than the first that
practically all the current will pass that way, leaving the first
circuit, to all intents and purposes, free. It is as if a farmer wished
to stop people passing across a certain field. Two methods would be open
to him: one to put up a high gate over which no one would dare to climb,
and the other to provide a short cut so much more pleasant and
convenient than the old path that no one having the choice of the two
ways would think of going the old way.
What the farmer would call a short cut the electrician calls a short
circuit, and a short circuit is often a more convenient way of cutting
off a current than a switch which interposes resistance. At all events,
in a case like this, a short circuit enables that to be accomplished
which would be very difficult by any other means.
In the apparatus as already described the battery had to drive the
current along a long wire, terminating at the distant receiving
instrument, whence the current returned via the earth. The foil and pen,
acting as a kind of electrical "tap," controlled this. When foil and pen
touched, the tap was open and current flowed. When the line of
non-conductive ink interposed itself, the tap was off and the flow
ceased.
But connect the battery directly to the wire, and place the foil and pen
in a short branch circuit, and the whole thing is reversed. Then the
opening of the "tap" sent current to the other end; now the opening of
the tap causes it to flow round the short branch and leave the main
wire. Then the closing of the tap stopped the current reaching the
farther end; now it causes it to do so. In fact, the entire action of
the apparatus is completely reversed, and the bare parts of the foil
become represented by blank paper, while the insulating lines produce
the marks. In short, a positive results instead of a negative.
Such was the scheme of Caselli years ago. It is mentioned here at some
length, since the principle of it is largely re-used in an improved
form in the most successful of modern apparatus for a like purpose.
It undoubtedly was a very excellent scheme, simple and effective, which
ought to have succeeded; but it did not do so, for the sufficient reason
that at that time knowledge of electricity and skill in constructing
delicate mechanism were not so highly developed as they are to-day. For
success, as has already been said, one thing was essential, and that
thing very difficult to obtain--a perfect synchronism between one stylus
and the other. If the one were but the slightest degree "out of step"
with the other, failure followed inevitably.
So the electrical transmission of sketches dropped for the time being.
More recently a perfectly successful solution of the problem has come in
another way altogether. This apparatus, at first called the
telautograph, but now known as the telewriter, it will be more
convenient to refer to later.
Of modern systems for the transmission of pictures the most successful,
probably, are the Korn telautograph and the Thorn-Baker telectrograph.
Both of these are able to transmit very fair reproductions of
photographs besides line drawings. The difficulty with photographs is,
of course, that many parts of them are not of equal blackness or
whiteness, but shade off gradually from one into the other. Take the
case of a simple portrait. Part of the subject's face will be pure
white, while the side in shadow will be comparatively dark. There is no
hard and fast line between the two, but by a gradation through an
infinite number of shades the one tones into the other. How can it be
possible to convey that, more or less mechanically, over a wire? The
solution is due to the fact that the eye will blend together a number of
distinctly different shades, if properly arranged, into a gradual
change. Really the change is step by step, but the effect is apparently
quite continuous. This can be seen in the "half-tone" illustrations in
this book. Close examination will show that such a picture is cut up
into small squares. In the pure white part the squares are invisible,
while in the perfectly black parts, if there be any, they are so merged
into one another as to be inseparable. But everywhere else in the
picture it will be seen that there are squares each with a dot in the
middle. In the darker parts the dots are large; in the lighter ones they
are small. We get the effect almost of colour, although the picture is
done entirely in black ink. The eye does not see the individual dots
when we are just looking at the picture; we have to examine it very
closely to find them. Yet they are there all the time, and it is simply
the peculiar action of the eye which sees beautiful half-tones, shading
imperceptibly one into another, whereas in real fact there are only a
vast number of equidistant dots, all equally black.
We see, therefore, that it is possible to split up a picture of any kind
into a number of small squares and to treat each square as being of
equal darkness throughout. Then, if we can communicate by wire that
particular degree of darkness to a distant station, where the small
parts can be put together in their proper order and given their correct
shade, the picture as constructed at the receiving end will be something
like that at the sending end. And we have only to make the size of each
separate square small enough to obtain a copy which will resemble the
original very closely indeed.
In the early days it was actually proposed to telegraph pictures by
ordinary telegraphy, using this principle. The suggestion was to agree
upon a code of twenty-six shades, each called by a letter of the
alphabet. One shade was to be _a_, the next _b_, and so on. Then the
picture was to be divided up into squares, and the particular shade of
each square telegraphed by means of the corresponding letter. The shades
thus communicated were to be put together at the receiving end, on a
prearranged system, and so the picture was to be built up. Given plenty
of time, that scheme might be moderately successful, but to get a really
good reproduction the subdivision needs to be so minute, and the number
of squares, therefore, so immense, that it would be quicker to send the
picture by train than to telegraph it by such laborious means. In a
fairly coarse half-tone block the squares are, say, 2500 to the square
inch. That number of letters would therefore have to be telegraphed for
every square inch of picture transmitted, to say nothing of the
difficulty of building up a picture of such a great number of parts and
giving to each the desired shade. That idea, abortive though it is in
its crude form, illustrates very clearly the fundamental principle on
which this work is done.
The problem is really to devise a machine which will do that same thing
rapidly and automatically divide up the original into a large number of
squares, and then send an electric current to represent each square,
such current by its strength to indicate the shade of the square: and
finally a similar instrument is needed to act as receiver, and to
reproduce those squares in the proper order, giving to each its correct
shade.
In practically all of them the mechanism is rotatory, the original being
placed upon a drum which turns round under a stylus, or its equivalent,
while the stylus gradually travels along from end to end after the
manner of the needle of a phonograph, or else the same result being
achieved by the drum itself having an endwise movement as well as a
rotative one. The receiving instrument is of similar form, and both must
start together, move at the same speed and indeed preserve a perfect
correspondence with each other.
If the distance be great between the two there may be difficulties due
to the "retardation" of the currents passing between them. Electricity
does not pass through long wires, particularly if they be under the sea,
with anything like the quickness which we are apt to think. Over a short
line and under favourable circumstances the receipt of a telegraph
signal at the farther end is practically instantaneous, but on long
lines, and under certain conditions, that is far from being the case.
Then something has to be done to quicken the action of the current, or
else the receiving drum must be made to lag behind the sending drum by
the requisite amount. In some cases, too, the transmitting apparatus
loses a little time in sending off the currents, and that, too, has to
be allowed for, so that, all things considered, the reader will see that
the successful solution of this problem is hedged about with many subtle
difficulties which are probably only appreciated by those who are well
acquainted by sad experience with the little vagaries of both
electricity and mechanical devices. Neither of them does quite what we
want it to do; each suffers from little faults, which in the case of a
delicate problem like this, where a difference of a hundredth of a
second would be fatal to success, introduce difficulties almost
insuperable.
To transmit line drawings, Professor Korn uses a sending instrument very
like that of Caselli. The picture is placed, either by hand or
photographically, upon a sheet of copper foil, which is fixed round the
rotating cylinder, the lines being formed of non-conducting material.
The foil being electrified and the stylus connected to the "line" or
main wire, currents pass to the farther end just as in the old
apparatus.
At the receiving end the drum is covered with photographic paper and
enclosed in a light-tight box. Through a hole in this box a fine pencil
of light passes from a lamp, suitable lenses being used to ensure that
the pencil shall have, as it were, a very fine point, producing a very
small spot of light upon the paper. If the light remains quite steady,
the drum meanwhile rotating, a line will be drawn by it upon the paper
which will be visible when the latter is developed. Since the drum not
only turns upon its axis, but also moves endwise one hundredth of an
inch at every revolution, this line will be a spiral, the turns of which
will be one hundredth of an inch apart. Thus the paper will be blacked,
practically uniformly, all over. Should the intensity of the light vary,
however, the line will at times be lighter than at others, while, should
it be cut off altogether for a moment, then there will be a
corresponding gap in the line, and it is easy to see that if these
lighter parts or gaps occur in the correct places they will form a
picture. In other words, by controlling that light we can build up a
picture upon the paper. The question is how to control it.
Professor Korn uses a form of the Einthoven galvanometer already
described. Instead of the silvered fibre generally employed in this
instrument, a silver wire is fitted, the movement of which partly or
entirely cuts off the pencil of light.
The Korn transmitter for photographs is quite different, although the
receiver is practically the same as what has just been described. The
basis of it is a peculiar power possessed by the metal selenium when in
a certain state. This, like all metals, is a conductor of electricity,
but of course offers resistance in some degree. Now the special feature
of selenium is that its resistance is reduced if light shine upon it.
Suppose, then, that current be flowing through a mass of selenium and
that the latter be suddenly illuminated brightly, the resistance will at
once fall and the current increase. On the other hand, should the light
falling upon the selenium diminish, its resistance will increase and the
current flowing through it will decrease. In short, given a suitable
arrangement, the current flowing in a circuit of which a selenium "cell"
forms a part will increase or decrease with the increase or decrease in
the light falling upon the cell.
A while ago the papers were telling striking stories of a way by which
blind people, so it was said, were to be recompensed for the loss of
their sight--a new sense, as it were, was to be given them by which they
could "hear" light, even if they could not see it. All this had
reference to this curious property of selenium, it being, of course, an
undoubted fact that it will vary an electric current in accordance with
the variations in the light, and if that current be led through a
telephone receiver a man, by holding that to his ear, could, in a sense,
hear the variations in the light.
[Illustration: THE TELEWRITER
This remarkable instrument transmits actual writing and drawings, the
receiving pen copying precisely the movements of the sending pen]
In the Korn transmitter for photographs selenium is employed as
follows:--A transparent photograph is made, on a celluloid or gelatine
film, and this is fixed upon a glass cylinder mounted as already
described. A pencil of light falls upon this in much the same way as in
the case of the receiver just described, and, as the cylinder revolves,
describes a fine spiral line all round and round it.
Moreover, the light passes right through the photograph and falls upon a
mirror inside, off which it is reflected on to a selenium cell. At every
moment, then, the light is falling upon some small part of the
photograph, and the amount of it which gets through and ultimately
reaches the selenium depends upon the density of that part.
Current, meanwhile, is flowing from a battery through the selenium, and
thence over the main wire to the distant station. As the light pencil
traces its spiral path over the rolled up photograph every variation in
the density of the picture is reproduced as a variation in the current
through the selenium. This, at the remote end, operates the Einthoven
galvanometer, the movements of which vary the shade of the spiral line
being drawn upon the photographic paper.
This process takes place with remarkable celerity, so that in a few
minutes the innumerable variations constituting a complete photograph
can be transmitted and faithfully recorded at the distant end of the
wire.
But perhaps the most successful of these methods is that known as the
telectrograph. It is surprisingly like the scheme of Caselli in
principle, and forms another example of the fact that good ideas often
fail through lack of the proper means to carry them out. Mr
Thorne-Baker, the inventor of the telectrograph, has had at his disposal
accumulated stores of knowledge and skill which did not exist in
Caselli's time. Consequently the former has made a brilliant success
where his predecessor produced only an interesting but somewhat
ineffective attempt.
Reference has been made already to the half-tone blocks wherein a host
of small dots of varying sizes make up a picture. Now instead of
parallel rows of dots parallel lines of varying thickness will give very
much the same result. The former are made by photographing the picture
through a sheet of glass ruled with two sets of lines at right angles to
each other. The latter can be made by using a screen with lines one way
only instead of two ways. It is therefore quite easy for a blockmaker to
produce a "process block" wherein lines are used instead of dots. For
this particular purpose, however, it is not an ordinary block that is
needed, although it is in essentials very similar. The picture to be
transmitted is photographed through a screen as if a half-tone block
were to be made. The negative so obtained is then printed by the gum
process on to a sheet of soft lead and, after washing, the picture
remains upon the lead in the form of lines of insoluble gum on a
background of bare lead. A squeeze in a press drives the gum into the
lead, and so gives the whole sheet a smooth surface over which a stylus
will ride easily, but which is, nevertheless, made up of conductive
parts and non-conductive parts, the latter forming the picture.
The lead sheet is then put upon a revolving cylinder and turned under a
moving stylus in the manner with which we are now familiar. The sheet is
placed with the lines lengthwise of the cylinder so that current passes
to the stylus except as it passes over the breadth of the lines, and so
similar lines are built up at the distant end.
The receiving mechanism is of the electro-chemical type which Caselli
used. The current passes from the receiving stylus to the paper, and
there makes its mark in a way that will be understood from the
description of the earlier apparatus.
The supreme advantage of this method of working, over that of Professor
Korn, is that the operator can see what he is doing. To obtain good
results, a number of electrical adjustments have to be made, and whether
he has got them right or wrong can be seen as soon as the picture begins
to grow upon the receiving paper. If a little readjustment be needed the
operator sees it and can set things right before the really important
part of the picture begins to appear, whereas with the Korn apparatus he
does not know what is happening at all, since he can see nothing until
the picture is finished and the photographic paper has been developed.
It will be apparent, too, to anyone who has carefully considered the
wireless telegraphy chapters, that it ought to be possible to make the
sending stylus or its equivalent control a wireless transmitter and a
wireless receiver to operate the receiving stylus, so as to be able to
send pictures by "wireless." Experiments to this end have been made with
some measure of success, and sooner or later we are almost sure to hear
that the difficulties, which are by no means small, have been overcome.
But we cannot conclude this chapter without a fuller reference to that
marvellous invention, the telewriter.
In this a man makes a sketch with a pen on a piece of paper, or maybe he
writes a message, and simultaneously a pen, hundreds of miles away if
need be, does precisely the same thing. The receiving instrument draws
the sketch line by line, or it transcribes the message in the actual
handwriting of the sender. A little touch, almost weird in its
naturalness, is that every now and then the receiving pen leaves the
paper and dips itself into a bottle of ink, after which it resumes its
work at the very spot where it left off.
Now how the complicated lines and curves, the strokes and dots which
make up a written language, even the little shakes and defects which
give each man's writing a personality of its own, how all these can be
sent over a wire is at first sight very difficult to understand. The
inventor of this apparatus has discovered an extremely simple way of
doing it.
But even he does not attempt to do it with one wire, it should be said,
for he uses two. This is no drawback when, as is often the case, it is
used in conjunction with a telephone, for the latter, to be effective,
also requires two wires. Years ago single wires were employed for
telephones as for telegraphs, the circuit being completed through the
earth. But the difficulty arose that every wire through which currents
flow is apt to induce currents in neighbouring wires--the induction coil
is based upon that fact--and so messages in one wire were overheard on
others, or, what was perhaps more annoying still, the dots and dashes
passing in a telegraph wire would produce loud noises in a telephone
wire that happened to be near. The use of two wires, however, entirely
removes that trouble, for the neighbouring current then induces two
currents instead of one, one in each, and it so happens that these are
opposed to each other, so that they neutralise each other. So every
telephone wire now is double and therefore is ready, as it were, to have
the telewriter fitted to it.
But even with two wires the difficulty seems insuperable until we
remember that the most complex of curves can be resolved into two simple
movements.
The sending pen, with which the original writing or drawing is done, is
attached to the junction of two light rods. The farther end of each rod
is attached to the end of a light crank fixed so that it can rotate or
oscillate, after the manner of cranks, in the plane of the desk upon
which the paper lies. All the joints mentioned are of the hinge nature,
so that as the pen is moved about the rods turn, more or less, one way
or the other, the two cranks. This simple mechanism, it will be
observed, carries out very effectively the principle just mentioned, for
it resolves the motion of the pen, no matter how complicated it may be,
into a simple rotating motion of the two cranks.
So the cranks turn this way or that as the draughtsman makes his
picture, and it is very easy to arrange that their movement shall vary
the strength of two electric currents, whereby we obtain electric
currents varying in accordance with the movement of the cranks.
This is done by making each crank operate a variable resistance or
rheostat. When in its extreme position on one side the crank permits
current to flow freely, but as it moves over to the other extreme
position the resistance in the path of the current is increased. Such an
arrangement is a common feature in electrical apparatus.
So current from a battery flows to the two wires leading to the distant
station, each passing through the rheostat connected to one of the
cranks. We may think of the rheostats as taps which can be turned on or
off by the action of the cranks. Let us imagine that crank _a_ is in the
position when the current flows freely--when the electrical "tap" is
fully open; then a strong current will flow along wire _a_, returning to
the sending battery via the earth. As that crank is moved the current
will gradually be reduced, until, if it be moved right over to the other
extreme, the current will be at its feeblest.
[Illustration: FIG. 15.--A Message received by Telewriter.]
Arrived at the other end, this current passes to a device which we may
describe simply as a magnet so arranged that its action pulls round a
crank against the restraining action of a spring.
Now the stronger the current the more does that magnet pull and the
farther does the receiving crank turn. The sending crank varies the
resistance, the resistance varies the current, the current varies the
strength of the receiving magnet, and the magnet varies the position of
the receiving crank. Properly adjusted, then, the motion of the crank at
the one end is communicated through that long chain of causes and
effects, until at last it is repeated _exactly_ by the movement of the
crank at the other end.
The same thing occurs simultaneously over each of the two wires, crank
_a_ at the sending end communicating over wire _a_ to crank _a_ at the
other end, while crank _b_ communicates its motion over wire _b_ to the
other crank _b_. Each sending crank is closely imitated in its every
action by the corresponding one at the distant station.
The two receiving cranks are connected by light rods to the receiving
pen in precisely the same way that the sending pen is connected.
Consequently, not only are the separate movements of the two cranks
repeated at the remote station but the complex movements of the sending
pen, which gave rise to the actions of the cranks, are also conveyed to,
and repeated by, the recording pen. The movements of the first pen are
resolved into rotating motions by the two cranks, these are transferred
to the other cranks, and their movements are in turn converted back into
the written curves.
Thus as the pen in the artist's hand draws his sketch, so does the
automatic hand at the other place, it may be at a great distance, repeat
faithfully his work, and the sketch grows line by line simultaneously at
both ends.
There is not space here to detail how, by another current superposed
upon those referred to already, the receiving-pen is made to dip itself
periodically into the inkwell at the will of the sender. By a cunning
use of alternating current this is done without in any way interfering
with the action of the cranks as described above.
But of course there is a severe limitation to the usefulness of this
machine, inasmuch as the drawing has to be made at the time of
transmission, and it can only be "put on the wire" by the hand of the
artist himself.