The advance of photography : its history and modern applications (1911)

PHOTO-TELEGRAPHY 365 arranged that the photograph used at the transmitting station is broken up like this half-tone process, and the receiving apparatus can be arranged so that the marks on the recorded image shall occupy the same relative positions, we have at once a means of adapting the above electrolytic process for the purpose of photo-telegraphy. Thorne Baker's method of doing this is as follows : At the transmitting station a metal drum is made to rotate under an iridium stylus. Printed in fish-glue upon this drum is a half-tone photograph prepared on lead foil. Every time the iridium stylus comes in contact with a clear part of the metal foil, a current passes through the wires to the receiver. This receiver consists of a rotating metal drum round which is placed chemically prepared paper, and over which a platinum stylus traces. Hence, when the current passes, a black or coloured dot or mark appears on the paper. The current will of course be interrupted for a time, the length of which will depend upon the width of the fish glue line. The width of the mark should of course depend on the duration of the current. A practical difficulty has been met with in connection with the marks obtained, on account of the very large number of signals which must be sent through per second, and the capacity of the line for long-distance wires. While each signal mark should be distinct and clearly defined, it was at first found that when messages are transmitted long distances, the marks obtained tail off into one another and a hopeless blur results. This difficulty has, however, been overcome by the introduction of a shunt circuit at the receiving station. This shunt circuit comprises two batteries, a variable resistance, and a variable condenser. By a suitable arrangement of the elements of this shunt circuit it is possible to obtain faithful copies of half-tone photographs by the telectograph method. In fact, as many as three