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
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 A2, 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
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.
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.