« PreviousContinue »
Wireless Control of Mechanisms
By Dr. Alfred Gradenwitz
Berlin Correspondent TECHNICAL WORLD MAGAZINE
VER since the discovery of wireless telegraphy, which, in so short a career, has assumed such universal importance, it has been the dream of inventors to transmit, without the agency of any conductive wire, not only telegraphic messages, but any kind of mechanical effects, governing, from a distance, all sorts of mechanisms. For instance, ability to control the motors propelling a vessel or an air-ship, or the engines of a power station from some distant point, would be an invaluable power. In order fully to understand
of a tube containing metal filings, which, when inserted in an electric circuit, will enable the presence of electric waves flowing through the space to be detected without any material connection with their starting point.
The tube used by Branly was a tube of some insulating material-usually glass-traversed by two conductive rods between which metal filings were inserted without any noticeable pressure. (See Fig. 1.) This tube was found to become conductive for electricity, like any metallic conductor, as soon as, at some distant point, an electric spark was made to pass between two metal balls, and to lose this conductivity under the action of a shock. Whenever such a tube is inserted in the
this problem, it will be well to recall the fundamental principle underlying wireless telegraphy.
While we are indebted to Marconi for its first practical realization, wireless telegraphy was invented in principle by Dr. Branly, of Paris. Several years before Marconi's successful work-in fact, as far back as 1890-Dr. Branly discovered the remarkable properties
circuit of an electric battery, the current will be arrested, until a spark is produced in the neighborhood. The current is then allowed to pass until the temporary conductivity of the tube is destroyed by a shock imparted to the tube, These phenomena are the starting points of wireless telegraphy.
The action of a spark, as shown by this "coherer" tube, spreads round the spark gap in all directions at the speed of light, traversing air as well as insulating bodies, pure water, partitions and walls. Metal covers, metallic solutions and salt water, however, intercept this action. The distance through which the action of the spark can be noted is some hundreds of feet, but it can be extended considerably by fitting a metal rod called "antenna" to the spark
gap. The action is due to the electric vibrations originated by the spark simultaneously with its characteristic light and sound effects. While the latter are perceived by the organs of our senses, we do not possess any organ capable of detecting electric vibrations and must re
FIG. 2. SENDING OPERATOR.
sort to artificial means, such as the coherer tube or radio-conductor above described. This plays the same rôle, then, in regard to electric vibrations, as does the eye in regard to light waves, and has been fitly termed the "Electric Eye." An improved type of radio-conductor is the tripod disc, designed by Branly and which consists of a brass plate having three feet, the lower points of which, (of polished steel), rest on a polished steel disc. When this is inserted in an electric circuit the current is interrupted by the imperfect contact between the points and the disc. The obstacle due to this contact is, however, overcome by the battery current as soon as a spark is produced by the induction coil of the sending station and the current continues to pass until its temporary conductivity is discontinued by a shock imparted to the disc.
The presence of the battery current is shown by the deflection of a galvanometer-needle. If now the galvanometer is replaced by an electro-magnet this will be energized as soon as the electric current is allowed to pass, thus attracting a soft iron plate. If this plate, pivoted upon a fixed axis, be so arranged as to mark by its swinging motion a dot on a tape of telegraph paper, the whole ap
paratus will constitute a Morse receiver, which registers messages in ordinary telegraph codes. If at the sending station, a spark be produced between the two balls of the induction coil, the coherer becoming conductive at the receiving station, the electro-magnet will be energized and the plate, being attracted, a dot will be marked on the tape. The shock of the plate against a stop will be sufficient to discontinue the conductivity of the radioconductor. Between the starting point, where the spark is produced, and the receiving station where the circuit of the radio-conductor is closed, there is no intermediary conductive wire.
From the above it will be readily understood why a radio-conductor should be able to produce at the distant station, without the intermediary of a wire, not only a deflection of a galvanometer or energizing of an electro-magnet but any effects of the electric current: incandescence of metal wires for electric lamps, lighting of electric arcs, illumination of Geissler tubes, production of X-rays, ignition of combustible bodies, explosion of mines, etc. It would also make possible production of the many mechanical effects at a distance: drilling of metal pieces, lifting of loads, etc.
Devices allowing these effects to be produced are actually employed in some
theatres, in order to control by the action of electric sparks many spectacular effects.
If a number of electric circuits, each controlling a given mechanical effect, are installed beforehand at the receiving station, each circuit having a special coherer, a single spark would simultaneously complete all the circuits thus producing all the effects at the same time. More complicated effects can however be produced by so arranging the several circuits as to have a single spark from the starting station control a first effect, this first effect controlling another, and so forth. Now as most effects produced in the circuit of the coherer itself, require a strong current, which would be apt to damage the coherer, it will in most cases be preferable to produce the action in a neighboring circuit, actuated by a relay disengaged by the coherer.
tion in question has been really produced at the distant station. This is effected by an ingenious outfit designed by Dr. Branly and of which a short description is given in the following:
At the receiving station, where the various effects to be produced have been arranged beforehand and where the services of no operator are required, there is installed a receiving apparatus which under the action of sparks given off at convenient intervals from the starting station, will control not simultaneously but successively, either a series of phenomena, being independent of one another or phenomena depending on each other. In the first case, Dr. Branly's apparatus affords a means of producing the several effects in any desired order, while in the latter, they may be produced in their proper order and discontinued in an inverted order.
At the starting station there is installed an operator acting on a transmitting apparatus by means of which sparks are given off. Being some
times as far distant from the receiving station as 200 miles, he is obviously not able to see the latter, but, controlling at will the phenomena to be produced at the receiving station, he is thereby kept posted, being able to check the effects produced by them as though they took place under his eyes.
The apparatus at the receiving station is to this effect provided with a horizontal cylindrical steel axle rotating slowly. In the first model constructed by Dr. Branly and which is represented in figure 3, the axle was driven by clock work, while in the second apparatus, which is better fitted for industrial use, a small electric motor has been
trolling another circuit, which contains the lamps in question, thus lighting these. After another revolution or any number of revolutions of the distributing axle the operator is in a position to put out the burning lamps by means of a spark from the starting station at the moment when the thickened sector of the same disc, by pressing on its contact rod, once more allows a current to traverse the circuit of the radio-conductor. Each of the discs thus constitutes a current interrupter controlling one of the effects to be produced. If there are four such discs it will be possible to produce four different effects. These effects may be independent of each other, being, for instance, the firing of a revolver, operation and stopping of a ventilating fan, lighting and extinction of
incandescent lamps, energizing of an electro-magnet able to lift an iron ball and to release it as soon as the magnetization ceases. While the above phenomena have been chosen for demonstration, their choice evidently is entirely arbitrary. The various functions of a complicated working machine could, for instance, be controlled by the spark.
During the fraction of a revolution of the axle while a given effect is being produced, none of the remaining effects can take place, all the remaining circuits being interrupted.
The operator at the starting station is enabled to ascertain whether the effect in question has been actually produced at the given moment, by means of an automatic wireless telegram starting from the receiving station and which is recorded automatically on a Morse receiver, the tape of which unwinds under the eyes of the operator at the starting station. The signals thus recorded are all due to sparks automatically produced by an induction coil at the receiving station, and this automatic wireless telegraphy is controlled by a special disc mounted on the distributing axle, and which is provided at its circumference with five sets of projecting teeth, the contacts of which, with convenient springs at each complete revolution of the axle, produce five sets of sparks separated by practically equal intervals of time. (Complete apparatus shown in Fig. 5.) The five sets of sparks record themselves on
the telegraph tape at the starting station, each interval left between two consecutive sets corresponding to a length of 4 inches through which the paper tape unwinds in the meantime. A sample of two such wireless messages received at the starting station during a complete revolution of the axis is represented below in figure 6. The Marconigram in question is as simple as possible, comprising only five sets of one, two, three, four and five sparks respectively, which sparks in each set are very close to each other. The interval between the first spark, which is simple, and the second, double, is called interval 1-2; let this be set apart for the firing of a revolver. The interval 2-3 between the double and triple sparks will correspond to the starting of the ventilating fan; the interval 3-4 to the lighting of the incandescent lamps, and the interval 4-5 to the energizing of the electro-magnet lifting the iron ball. In these intervals take place, one after the other, the effective contacts of the thickened sectors with their contact rods, closing the circuits for the various effects, all of which circuits are open during the sparking intervals, that is during the short fraction of a revolution when the sparks are produced. During the contact intervals, the operator, while keeping his eyes fixed on the unwinding paper tape, will, by means of a key, cause the sparks to pass in the induction coil. The interval 5 to 1 corresponds to the electro-motor substituted for the clock