Wireless Transmission of Photographs / Second Edition, Revised and Enlarged 1919

by Marcus J. Martin

The switch D is then closed, and the arm of the switch D¹ placed on the contact stud (1), at the transmitting station only. As soon as the switches are closed the clutch F comes into action, and the transmitting machine begins to revolve. When the whole of the line print wrapped round the drum of the machine has passed under the stylus, the end of the shaft D, Fig. 36, engages

with the spring m, breaking the clutch circuit and allowing the motor to run free. As soon as the machine stops, the switch D is opened and the machine run back to its starting position by hand.

At the receiving station the switch D is also closed, and the arm of the switch D¹ placed on the contact stud (2). The closing of these switches does not bring the clutch F into operation until current from the telephone relay U connected to the wireless receiving apparatus works the sensitive polarised relay K, which in turn completes the circuit of the circuit-breaker L. When the armature of L is attracted, the circuit of the relay K is broken, the circuit of the clutch F is completed, and the machine starts revolving.

The current from the relay U, due to the transmitting stylus passing over one contact strip on the metal print, is too brief to actuate the heavier mechanism of the relay K, hence the need of the margin of bare foil at the commencing edge of the metal print, so that a practically continuous current will flow to the relay K until the armature is attracted. As, however, the relay is not actuated at the receipt of the first signal, and as it is necessary for the machine to start recording at a certain point on the film, viz.

at the edge of the lap—the reason for this was given in Chapter IV.—the starting position of the receiving drum will be similar to that given in the diagram Fig. 52, where X indicates the lap of the photographic film, and the arrow the direction of rotation.

It is, of course, obvious that a somewhat similar adjustment must be made with regard to the position of the stylus on the metal print at the transmitting machine.

In the present system, as in almost every photographic method of receiving that has been described, the Nernst lamp is invariably mentioned as the source of illumination. Since the advent of the high-voltage metal-filament lamps the Nernst lamp has fallen somewhat into disuse for commercial purposes, but it possesses certain characteristics that render it eminently suitable for the purpose under discussion.

The main principle of this type of lamp depends upon the discovery made by Professor Nernst in 1898, after whom the lamp is named, that filaments of certain earthy bodies when raised to a red heat became conductive sufficiently well to pass a current which raised it to a white heat, and furthermore that the glowing filament emitted a brighter light for a given amount of current than carbon filaments.