(2) Prof. Korn's more recent apparatus is his telautograph, by means of which half-tone photographs prepared with a single-line screen can be telegraphed. The photograph is printed in fish-glue on a thin copper sheet, developed, and attached to the metal drum, D, in diagram C.
A metal stylus s traces a spiral path over the photograph, and as it travels over each glue line the flow of current from a battery, E, through the cylinder and stylus is interrupted. The same form of receiving apparatus is used, but a great many improvements have been made in the galvanometer used, with the result that a picture 7 in. by 5 in. can be telegraphed in a few minutes, with as many as 15o lines to the inch.
T. Thorne Baker's telectrograph has been largely used commercially between Manchester, London and Paris. Its great advan tage over other systems is that it does away with the reception on a photographic film. The whole operation, therefore, is conducted in full light, and the image can be watched during its entire reception. A portable transmitter was designed in 1910 by the inventor, and this was used between Brighton and London in that year. The system is shown diagrammatically at D. A and s are the metal cylinders of the transmitter and receiver.
A is provided with a steel stylus s, and s with a platinum stylus T. A half-tone single-line photo graph printed on lead foil and pressed perfectly smooth is attached to A, and a piece of chemically prepared paper is wrapped round B. The battery B supplies the electric current. When ever the steel stylus is in contact with the metal of the picture, that is, when it is not travelling over a glue line, current flows through the wires to the receiver, and a black dot appears under the stylus T. The original picture is thus reproduced, dot for dot, at the receiving machine, and a picture 8 in. by 5 in. can be transmitted in from ten to fourteen minutes. The system is explained here in its simplest form ; in practice, the receiv ing apparatus is complicated, owing to the fact that the electric currents, in passing through long distance wires, become lengthened in period and changed in intensity, with the effect of distorting and blurring the received photograph. The various difficulties have been overcome by means of the line balancer invented by T. Thorne Baker, to which the success of a system based on many earlier attempts has been due.