SECRETS OF WIRELESS TELEGRAPHY.

Progress, Volume II, Issue 6, 1 April 1907, Page 222

SECRETS OF WIRELESS TELEGRAPHY.

By Dr. Alfred Gradenwitz Berlin Correspondent, Technical World Magazine.

The greatest drawback to the general adoption of wireless telegraphy, as is well known, is the fact that the electric waves being propagated in all directions, there is a possibility of a third party interfering with the transmission of messages between two stations or intercepting the telegrams. The " tuning " methods so far suggested, fail entirely to solve the problem. Prof. Ferdinand Braun of Strassburg, Alsatia, has for many years carried on experiments intended to discover a suitable method of avoiding this inconvenience ; and these experiments have at last resulted m what bids fair to prove a successful solution of the problem. The simplest method would evidently be to give off waves in only one direction, just as light rays are directed by projectors. Though this process was a few years age suggested by a German engineer (Mr. Blochmann), no recorded demonstration of its practicability has so far been forthcoming. Professor Braun's method is based on the production of rapid electrical oscillations showing a displacement in phase corresponding to that in the acoustical problem of generating two vibrations which shall be of exactly the same pitch, but of which one shall part by a fraction of a millionth of a second earlier than the other. He succeeded in so adjusting to each other two vibration circuits each being charged, and, as it were, waiting for the discharge to be started by means of a spark — that the sparks ordinarily so capricious can with absolute certainty be made to start as may be desired, either at exactly the same moment or at exceedingly small intervals. After developing this method theoretically and m the laboratories of Strassburg University in conjunction with his assistants, Professor Braun proceeded to apply his results to a comparatively Simple practical problem, with a view of ascertaining whether or not the tests would agree with the results of calculation. These tests were carried out on one of the Strassburg shooting grounds. The sending fetation includes three wooden poles arranged >at distances of about 100 ft., located at the angles of an equilateral triangle. An insulated copper wire is carried up to the top of each of the poles, the lower ends of which wires are united in the central cabin containing the apparatus whence the wires are supplied with electrical oscillations. The principle of this arrangement may be explained as follows : Suppose the two front wires (1, °) to begin vibrating simultaneously, while the back wire (3) begins giving off its electrical waves somewhat later ; suppose, further, the recerv ing stations to be situated somewhere on a line drawn from 3 and extended through the middle point ot the line connecting 1 and 2. Now, if waves are given off in the direction of the receiving station, the following conditions may be presumed : The sender 3 will begin giving off its waves somewhat later than 1 and 2 these waves thus lagging somewhat behind those starting from 1 and 2. As they have, moreover, to traverse the distance from 3 to th« pline 1 2, before reaching the waves starting from 1 .and 2 another difference in time will be added : and the total difference may be so calculated that the wave starting from 3 will exactly interfere with those given off from 1 and 2, weakening the time, or entirely suppressing them in case the intensity of wave 3 is equal to the aggregate intensity of waves 1 and 2, in which case a station situated in the direction referred to will receive no signals. If, on the other hand, sender 3 begins vibrating earlier than senders 1 and 2, the waves may be made to assist one another, and in this case the receiving station will receive signals. This affords a ready means of either sending messages to the receiving station or eliminating the latter from the telegraph service, as may be desired. In order to illustrate the almost infinitesimal time differences concerned, it may be said that in connection with Professor Braun's experiments, the sender 3 began oscillating by about one 10-mil-lionth part of a second earlier or later than the senders 1 and 2. He thinks a difference can be accurately maintained up to one 20-milhonth part of one second in 6 years. The three-pole arrangement is obviously only a special simplified case of a general arrangement in a given figure, enabling the time differences to be accurately calculated The limitation to three symmetrically distributed wires, outside of its simplicity, has, however, a further advantage The direction of maximum activity in space may obviously be turned round, placing it at right angles to any one of the edges of the triangle. Instead of starting the radiation from 3 across 1, 2, it may

be started from any one of the three angles across the respective opposite sides, by simply exchanging the mutual role of the poles, which is effected by simply turning a crank in the apparatus cabin. The whole may thus be said to work like what might be compared to a blinking fire, being rotated by steps of 120 degrees each, while immediate steps may be inserted. By means of an accurate calculation and graphical representation, the maximum effect is shown gradually to decrease towards both sides of the triangle. In spite, therefore, of considerable dispersion, there is thus a large angular space practically free from any effects — an electrical shade space. In connection with the tests above referred to, the receirer was placed over 1000 yards, at which distance measurements of the receiving intensity could be made by direct methods. The favourable results obtained can, it is claimed, be extended to the case of great distances. Professor Braun's method thus affords a possibility of designing on paper such arrangements as will comply with any given conditions as to effects in space. Choosing, for instance, five points of convenient arrangement, which are excited in a proper way, the effect in a given direction may obviously be concentrated much more strongly than in connection with the 3-point arrangement above referred to, while no radiation is given off towards the back. This opens up a practically unlimited series of possibilities of designing directed sending outfits, all of which allow of a rotation as in the case of the three-wire arrangement.