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485. Proof that the discharge of a Leyden jar is oscillatory. We found in § 408, p. 346, that the sound waves sent out by a sounding tuning fork will set into vibration an adjacent fork, provided the latter has the same natural period as the former. Following is the complete electrical analogy of this experiment.



Let the inner and outer coats of a Leyden jar A (see Fig. 455) be connected by a loop of wire cdef, the sliding crosspiece de being arranged so that the length of the loop may be altered at will. Also let a strip of tin foil be brought over the edge of this jar from the inner coat to within about 1 millimeter of the outer coat at C. Let the two coats of an exactly similar jar B be connected with the knobs n and n' by a second similar wire loop of fixed length. Let the two jars be placed side by side with their loops parallel, and let the jar B be successively charged and discharged by connecting its coats with a static machine or an induction coil. At each discharge of jar B through the knobs n and n' a spark will appear in the other jar at C, provided the crosspiece de is so placed that the areas of the two loops are equal. When de is slid along so as to make one loop considerably larger or smaller than the other, the spark at C will disappear.

FIG. 455. Sympathetic electrical vibrations


The experiment therefore demonstrates that two electrical circuits, like two tuning forks, can be tuned so as to respond to each other sympathetically, and that just as the tuning forks will cease to respond as soon as the period of one is slightly altered, so this electric resonance disappears when the exact symmetry of the two circuits is destroyed. Since, obviously, this phenomenon of resonance can occur only between systems which have natural periods of vibration, the experiment proves that the discharge of a Leyden jar is a vibratory, that is, an

oscillatory, phenomenon. As a matter of fact, when such a spark is viewed in a rapidly revolving mirror, it is actually found to consist of from ten to thirty flashes following each other at equal intervals. Fig. 456 is a photograph of such a spark.


In spite of these oscillations the whole discharge may be made to take place in the incredibly short time of 1,000,000 of a second. This fact, coupled with the extreme brightness of the spark, has made possible the surprising results of so-called instantaneous electric-spark photography. The plate opposite page 425 shows the passage of a bullet through a soap bubble. The film was rotated continuously instead of intermittently, as in ordinary moving-picture photography. The illuminating flashes, 5000 per second, were so nearly instantaneous that the outlines are not blurred.

486. Electric waves. The experiment of § 485 demonstrates not only that the discharge of a Leyden jar is oscillatory but also that these electrical oscillations set up in the surrounding medium disturbances, or waves of some sort, which travel to a neighboring circuit and act upon it precisely as the air waves acted on the second tuning fork in the sound experiment. Whether these are waves in the air, like sound waves, or disturbances in the ether, like light waves, can be determined by measuring their velocity of propagation. The first determination of this velocity was made by Heinrich Hertz (see opposite p. 102) in 1888. He found it to be precisely the same as that of light, that is, 300,000 kilometers per second. This result shows, therefore, that electrical oscillations set up waves in the ether. These waves are now known as Hertzian waves.

FIG. 456. Oscillations of the electric spark

The length of the waves emitted by the oscillatory spark of instantaneous photography is evidently very great, namely, about 300.000.000 30 meters, since the velocity of light is



300,000,000 meters per second, and since there are 10,000,000 oscillations per second; for we have seen in § 382, p. 323, that wave length is equal to velocity divided by the number of oscillations per second. By diminishing the size of the jar and the length of the circuit the length of the waves may be greatly reduced. By causing the electrical discharges to take place between two balls only a fraction of a millimeter in diameter, instead of between the coats of a condenser, electrical waves have been obtained as short as .3 centimeter, only ten times as long as the longest measured heat waves.

487. Detection of electric waves. In the experiment of § 485 we detected the presence of the electric waves by means of a small spark gap C in a circuit almost identical with that in which the oscillations were set up. The visible spark may be employed for the detection of waves many feet away from the source, but for detecting the feeble waves which come in from a source hundreds or thousands of miles away we must depend upon sounds produced in an extremely sensitive telephone receiver, as explained in the next section.

488. Wireless telegraphy. Commercial wireless telegraphy was realized in 1896 by Marconi (see opposite p. 316), eight years after the discovery of Hertzian waves. The essential elements of a tuned wave-train, or "spark," system of wireless telegraphy are as follows:

The key K at the transmitting station (Fig. 457, (1)) is depressed to allow a current from the alternator A to pass through the primary coil P of a transformer T1, the frequency of the alternations in practice being usually about 500 cycles per second. The high-voltage current induced in the secondary S charges the condenser C1 until its potential rises high enough to cause a spark discharge to take place across the gap s. This discharge of C1 is oscillatory (§ 485), and the oscillations thus produced in the condenser circuit containing C1, s, and L1 may, in a low-power short-wave transmitting set, have a frequency as high as 1,000,000 per second. An oscillation frequency much lower than this is generally used and is subject to the control of the operator through

the sliding contact c, precisely as in the case illustrated in Fig. 455. The oscillations in the condenser circuit induce oscillations in the aërialwire system, which is tuned to resonance with it through the sliding contact c'.

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FIG. 457. Transmitting and receiving stations for wireless telegraphy

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Receiving Station

As long as the key K is kept closed (assuming a 500-cycle alternator to be used), 1000 sparks per second occur at s, and therefore a regular series of 1000 wave trains (Fig. 458) pass off from the aërial every econd and move away with the velocity of light. If the oscillations which produce a wave train have a frequency of, say, 500,000 per second, each wave in the

wave train has a length of

300,000,000 500,000 600 meters; and if these wave trains are produced at the rate of 1000 per second, they follow each other at regular distances of 300,000 meters, that is, nearly 200 miles.

The waves sent out by the aërial system of the transmitting station induce like oscillations in the distant aërial system of the receiving station (Fig. 457, (2)), which is tuned to resonance with it. In case the receiving aërial must be tuned to respond to very long waves, the switch O is closed to cut out the condenser C,, and the inductance, or loading coil, B1 is used; whereas, to tune to very short waves, the switch O is opened and the variable

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FIG. 458. One wave train from oscillatory discharge



One of the most notable developments of the war was the directing of a squadron of airplanes in intricate maneuvers by wireless telephone either from the ground or by the commander in the leading plane. The upper panel shows the pilot and the observer conversing with special apparatus designed to eliminate plane noises, and the lower panel shows President Wilson talking by wireless to airplanes

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