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

366 THE ADVANCE OF PHOTOGRAPHY hundred sharply denned marks can be recorded per second. In practice, the receiving drum is made to rotate a little quicker than the drum on which the photograph to be transmitted is placed. The speed of rotation is about thirty per minute. As the receiving drum completes its rotation before the transmitter, it is stopped by a steel check and is obliged to wait until the other drum has caught it up. When the transmitting drum has completed its turn, a transitory current is sent to the receiving instrument ; this is led into a relay which actuates an electro-magnet, and this magnet removes the check. By this arrangement, no matter how much one drum gets out of step with the other, the fault is limited to each rotation, for both drums must alwa}rs start on a new rotation at the same instant. The great advantage of this method over the two previously described, is that while in those cases it is necessary to develop the film or bromide paper before one is able to tell whether a satisfactory picture has been received, by this method the picture can be watched as it is being gradually built up. The telephone lines need then only be held until the operation of receiving the picture is completed, when the telectograph method is employed, so the expense incurred by holding the lines until the photograph has been developed is avoided. Fig. 164 shows a photograph which has been transmitted by this process. Portable sets of apparatus have been constructed by Sanger-Shepherd, and it is believed that a much more extended use will be made of this method of transmitting photographs. Some attempts have been made to send photographs by the aid of wireless telegraph}7, and, although we can only look upon this as being in a more