Patent GB-190820638-A (GB)
Title: Improvements in and relating to Secrecy Systems for Wireless Communication.
Inventor: FOREST LEE DE
Filed: 1908-10-01
Granted: 1909-06-03
Abstract: 20,638. Forest, L. de. Oct. 1. Wireless telegraphy.-In a secrecy system of wireless telegraphy, the signals are transmitted and received by means of two or more wave trains of different frequencies, whose amplitudes are continuously varied between a maximum and a minimum, as shown in Fig. 4, so as to produce low wave-group frequencies to which the detecting instrument can respond. During transmission, one of the wave trains continuously modified in this manner is being emitted, and signalling is effected by the interruption of the other wave train iu the usual manner. The lower or group frequencies of the two trains are in the ratio of 2: 1 or 1 :1, and the phase relationship between those two groupfrequencies is variable at will. At the receiving- station two oscillation circuits are tuned to the oscillation frequencies of the two transmitted wavetrains, so that, when signals are being transmitted, currents corresponding in frequency and phase relation to the wave groups flow in the two detector circuits. In these detector circuits are solenoids the cores of which carry mirrors 40, 41, Fig. 2, and vibrate in accordance with the currents flowing in the coils. A ray of light reflected from the two mirrors falls on a screen and traces a signalcurve there, the shape of which depends on the ratio of the frequencies and on the phase relation of the currents flowing in the detector circuit. To obtain further secrecy, the transmitting operator, while working with a given group, frequency ratio, and phase relation, may depress in any manner the keys corresponding to the other frequency and to the other phase relations to send blind signals so that a number of different figures will constantly appear on the receiver screen, for one only of which the receiving operator must watch. A signal may be given from time to time to change from one light-figure to another. To prevent the distraction of the attention of the receiving operator by the blind signals, a movable screen having apertures corresponding to the shape of the various lightfigures may be used to cut off those blind signal figures. The means for varying the amplitude of the transmitted waves comprises rotating resistance disks I' ... I', Fig. 3, connected in the antenna citcuits. The disks are made of graphite mixed, if desired, with copper or insulating-material, and are so constructed that their resistance between the spindle and a brush bearing on the circumference varies from a minimum at 35 to non-conductivity at 36. Two sets of such discs are mounted on two shafts rotating at the same speed. Either disk of the one set I<1>, I<2> may be connected by means of the keys K<1>, K<2> to the constantly oscillating antenna circuit, while any of the other set P ... I<7> may be connected in the other circuit by means of the transmitting-keys K<3> ... K'. The disk I' has twice as many non-conducting sections as the others, so as to give a double group-frequency, while the disks I<3> ... I<7> are all set at different angles in order that any desired phase-relation between the wave-groups may be chosen.