CHAPTER XIX
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.