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.