Marvels of Scientific Invention Part 15

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 pa.s.s in an appreciable quant.i.ty, and the other is to provide a second path for the current so much easier than the first that practically all the current will pa.s.s that way, leaving the first circuit, to all intents and purposes, free. It is as if a farmer wished to stop people pa.s.sing 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" ill.u.s.trations 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 coa.r.s.e 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, ill.u.s.trates 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 "r.e.t.a.r.dation" of the currents pa.s.sing between them. Electricity does not pa.s.s 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 circ.u.mstances 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 pa.s.s 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 pa.s.ses 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 ma.s.s 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.

[Ill.u.s.tration: 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 gla.s.s 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 pa.s.ses 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 const.i.tuting 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 acc.u.mulated 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 gla.s.s 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 pa.s.ses to the stylus except as it pa.s.ses 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 pa.s.ses 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 pa.s.sing 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 pa.s.sing 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.

[Ill.u.s.tration: FIG. 15.--A Message received by Telewriter.]

Arrived at the other end, this current pa.s.ses 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.