Chapter III.) which can record signals over a length of line in which the

capacity effects are very slight, with the marvellous speed of 12,000 a minute, but this speed rapidly decreases with an increase of distance between the {5} [Illustration] two stations. The effect of capacity upon an intermittent current is clearly shown in Fig. 1. If we were to send twenty brief currents in rapid succession over a line of moderate capacity in a given time, we should find that instead of being recorded separately and distinctly as at _a_, each mark would be pointed at both ends and joined together as shown at _b_, while only perhaps fifteen could be recorded. If the capacity be still farther increased as at _c_, only perhaps half the original number of currents could be recorded in the same time, owing to the fact that with an increase of resistance, capacity, and inductance of the line a longer time is required for it to charge up and discharge, thereby materially lessening the rate at which it will allow separate signals to pass; the number of signals that can therefore be recorded in a given time is greatly diminished. If we were to attempt to send the same number of signals over a line of great capacity, as could be sent, and recorded separately and distinctly over a line of small capacity--the time limit being of course the same in both instances--we should find that the {6} signals would be recorded practically as a continuous line. The two latter cases _b_, and _c_, Fig. 1, clearly shows the retardation that takes place at the commencement of a current and the prolongation that takes place at the finish. If the photo-telegraphic system previously mentioned could be rendered sensitive enough to work on the Atlantic cables, we should find that only about 1200 signals a minute could be recorded, and this would mean that a photograph which could be transmitted over ordinary land lines in about ten minutes would take at least fifty minutes over the cable. This would be both costly and impracticable, and time alone will show whether, for long-distance work, transmission by wireless will be both cheaper and more rapid than any other method. At present wireless telegraphy has not superseded the ordinary methods of communicating over land, but there can be no doubt that wireless telegraphy, if free from Government restrictions, would in certain circumstances very quickly supersede land-line telegraphy, while it has proved a formidable commercial competitor to the cable as a means of connecting this country with America. Likewise we cannot say that no system of radio-photography will ever come into general use, but where there is any great distance to be bridged, especially over water, wireless transmission is really the only practical solution. From the {7} foregoing remarks, it is evident that a reliable system of radio-photography would secure a great victory in the matter of time and cost alone, besides which, the photo-telegraphic apparatus would be merely an accessory to the already existing wireless installation.

There have been numerous suggestions put forward for the wireless transmission of photographs, but they are all more or less impracticable. One of the earliest systems was devised by de' Bernochi of Turin, but his system can only be regarded interesting from an historical point of view, and as in all probability it could only have been made to work over a distance of a few hundred yards it is of no practical value. Fig. 2 will help to explain the apparatus. A glass cylinder A' is fastened at one end to a threaded steel shaft, which runs in two bearings, one bearing having an internal thread corresponding with that on the {8} shaft. Round the cylinder is wrapped a transparent film upon which a photograph has been taken and developed. Light from a powerful electric lamp L, is focussed by means of the lens, N, to a point upon the photographic film. As the cylinder is revolved by means of a suitable motor, it travels upwards simultaneously by reason of the threaded shaft and bearing, so that the spot of light traces a complete spiral over the surface of the film. The light, on passing through the film (the transmission of which varies in intensity according to the density of that portion of the photograph through which it is passing), is refracted by the prism P on to the selenium cell S which is in series with a battery B and the primary X of a form of induction coil. As light of different intensities falls upon the selenium cell,[2] the resistance of which alters in proportion, current is induced in the secondary Y of the coil and influences the light of an arc lamp of whose circuit it is shunted. This arc lamp T is placed at the focus of a parabolic reflector R, from which the light is reflected in a parallel beam to the receiving station.

The receiver consists of a similar reflector R' with a selenium cell E placed at its focus, whose resistance is altered by the varying light falling upon it from the reflector R. The selenium cell {9} E is in series with a battery F and the mirror galvanometer H. Light falls from a lamp D and is reflected by the mirror of the galvanometer on to a graduated aperture J and focussed by means of the aplanatic lens U upon the receiving drum A^2, which carries a sensitised photographic film. The two cylinders must be revolved synchronously. The above apparatus is very clever, but cannot be made to work over a distance of more than 200 yards.

A system based on more practical lines was that invented and demonstrated by Mr. Hans Knudsen, but the apparatus which he employed for receiving has been discarded in wireless work, as it is not suitable for working with the highly-tuned systems in use at the present time.

Knudsen's transmitter, a diagrammatic representation of which is given in Fig. 3, consists of a flat table to which a horizontal to-and-fro motion is given by means of a clockwork motor. Upon this table is fastened a photographic plate which has been prepared in the following manner. The plate upon which the photograph is to be taken has the gelatine film from three to four times thicker than that commonly used in photography. In the camera, between the lens and this plate, a single line screen is interposed, which has the effect of breaking the picture up into parallel lines. Upon the plate being developed and before it is {10} [Illustration] completely dry, it is sprinkled over with fine iron dust. With this type of plate the transparent parts dry much quicker than the shaded or dark parts, and on the iron dust being sprinkled over the plate it adheres to the darker portions of the film to a greater extent than it does to the lighter portions; a picture partly composed of iron dust is thus obtained. A steel point attached to a flat spring rests upon this plate and is made to travel at right angles to the motion of the table. As the picture is partly composed of iron dust, and as the steel needle is fastened to a delicate spring it is evident that as the plate passes to and fro under the needle, both the spring and needle are set in a state of vibration. This vibrating spring makes {11} and breaks the battery circuit of a spark coil, which in turn sets up sparking in the spark-gap of the wireless apparatus.

The receiver consists of a similar table to that used for transmitting, and carries a glass plate that has been smoked upon one side. A similar spring and needle is placed over this plate, but is actuated by means of a small electro-magnet in circuit with a battery and a sensitive coherer. As the coherer makes and breaks the battery circuit by means of the intermittent waves sent out from the transmitting aerial, the needle is made to vibrate upon the smoked glass plate in unison with the needle at the transmitting end. Scratches are made upon the smoked plate, and these reproduce the picture on the original plate. A print can be taken from this scratched plate in a similar manner to an ordinary photographic negative.

The two tables are synchronised in the following manner. Every time the transmitting table is about to start its forward stroke a powerful spark is produced at the spark-gap. The waves set up by this spark operate an ordinary metal filings coherer at the receiving end which completes the circuit of an electro-magnet. The armature of this magnet on being attracted immediately releases the motor used for driving, allowing it to operate the table. The time taken to transmit a photograph, quarter-plate size, is about fifteen minutes. {12} Although very ingenious this system would not be practicable, as besides speed the quality of the received pictures is a great factor, especially where they are required for reproduction purposes. The results from the above apparatus are said to be very crude, as with the method used to prepare the photographs no very small detail could be transmitted.

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