Early in July some experiments were conducted at Strasburg by Dr. Ferdinand Braun with a system of directed wireless telegraphy which he has invented. The results seem to him very promising, for he was able at will to direct the waves so as to actuate the receiver at the receiving station or not. Dr. Braun in several German journals discusses his experiments and points out the probable usefulness of his developments.
Since electric waves are governed by the same laws as light waves, it should be possible to throw a beam in one direction by means of a parabolic reflector, but the practical difficulties in the way seem to be insuperable. It occurred to Dr. Braun, however, that he could construct a system of sending wires which could be made to intensify the wave in one direction and interfere with it in another. If two sending wires are tuned to exactly the same pitch and are operated by the same exciting apparatus, but are so arranged that one of them will be set in vibration a small fraction of a second later than the other, it should be possible to obtain interference. The difficulties encountered in doing this are those of tuning two oscillators to the same pitch, and of producing the desired difference in phase.
New methods had to be devised for measuring these exceedingly small time differences. This has been done, but the method is not described. Extended lab oratory experiments resulted in the development of means of tuning the wires and for producing the dif. ference in phase sought without throwing the two wires out of tune. In this work Dr. Braun was assisted by Dr. Papalexi, of the Strasburg Institute, and Dr. Mandelstam, who co-operated with him.
The laboratory results agreeing satisfactorily with the theory, it remained to test the system on a practical scale. Fortunately a suitable place for carrying out] this work on a larger scale was available in the large drill ground at Strasburg, known as the Polygon which was courteously tendered to the experimenters by the military authorities. The investigations were carried out on a small scale as compared with wireless telegraph transmission systems, since it was desirable to obtain quantitative, and not merely qualitative, results. It was intended to measure the difference in intensity of the waves sent out in different directions and for this purpose a relatively short transmission was advantageous.
The sending sta:ion consisted of three wooden masts arranged at the corners of an equilateral triangle. Upon each of these masts was stretched a wire or antenna, from the lower point, of which a connecting wire was carried to wooden building placed at the center of the triangle, in which are mounted the various sending devices. A sketch of the arrangement is given in the accompanying figure. By means of the apparatus contained in the building it was possible o set the wires on posts 1 and 2 into synchronous vibration, and to set up a vibration of the wire on post 3 of the same pitch, but which lagged or led in phase the vibrations of the other two. Assuming the vibration of wire 3 to be slightly lagging, interference between it and that of wires 1 and 2 would take place in the direction from 3 over the house at right angles
to the plane containing posts 1 and 2. This amounts to throwing an electrical shadow in that direction. If, on the other hand, the vibrations of wire 3 were made to lead those of 1 and 2 by a proper amount, an amplified wave would be sent out in the same direction, and a shadow thrown backward from wire 3 and, at the same time, interference on the sides would take place. In carrying out this work it was necessary to obtain great accuracy in timing the vibrations of the three wires. The time differences was about one ten-milMonth part of a second. Dr. Braun concludes from his work that it is possible to adjust the time difference to within a two-hundred-millionth part of a second. This amounts to an accuracy of one second in six years.
This system of three masts is only a simple arrangement, for more sending wires might be used with an augmented effect; but for the work in hand the simplicity of the system presented some important advantages. It should be noted that transmission is not limited to one direction only with three masts, for it can be sent in the reverse direction by reversing the phase relations. By alternating the function of wire 3 with that of wires 1 and 2 it is possible to transmit messages at will in six directions. The simple addition of a commutating switch in the sending station enables this change in direction to be made easily and at will. It was found that the receiving apparatus at a particular position responded without fail when this commutating switch was in one pesition, and yet showed no signs when the position of the switch was changed. The distance of the sending station was in these researches 1.3 kilometers.
The results of the experiments were completely in accord with the laboratory investigation and with theory. In other words, it is possible by means of this arrargement to direct a wireless message through a fairly narrow angle. Dispersion takes place and is not small, but there is a decided electrical shadow in the reverse direction, and there is, in fact, a wide angle in which no effect can be measured.
If the receiving station had been equipped for transmitting messages also, it would have had three or more antennae, and there is no doubt that these wires could be used in an analagous way for increasing the effect at that point.
The use of more than three wires would enable the transmission to be sent out through a still smaller angle with a less degree of dispersion; but it would complicate the system somewhat. The system as it is, with the three wires, is considerably more complicated than that having but a single wire, and the question is raised whether a parabolic reflector consisting of a number of wires properly arranged might not be employed with but a single oscillating wire at the focus. This is shown to be impracticable, since to be of any value with waves of the length of 120 meters, which is comparatively short, and therefore relatively favorable for this purpose - the sending wire should be about thirty meters from the reflector, and the latter should have an opening of 120 meters at the focus. Moreover, this reflector should be extended many hundred meters at the sides in order to prevent dispersion. A reflecting wire placed at this distance from the sending wire occupies a very small angle, and hence would be almost useless as a reflector.
It was found that the three wires, as used in the interference system, had no effect whatever as a reflector. Although there is no fundamental reason why this difficulty should not be overcome, there is the other objection that the parabolic reflector should be about a kilometer long to give results comparable with those obtained with the three-wire system described.
The Strasburg tests, in Dr. Braun's opinion, demonstrate that a directed system of wireless telegraphy is practicable. Five wires might be substituted, which would increase the action in one direction and give rise to less dispersion, but the three-wire system gives results as favorable as could be expected. - " Electrical Review. "