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The Marvels of Modern Physics


By Rogers D. Rusk, B. Sc.




High Frequency Phenomena

Currents of high frequency and high potential exhibit many phenomena which are entirely different from those of ordinary currents.

A discharge which would ordinarily be fatal, will not even be felt by the body if it is of high frequency. An incandescent lamp may be short-circuited by a heavy copper bar and yet such a current will light it, while vacuum tubes may be caused to glow brilliantly without any metal connections at all. This, of course, does not at first seem reasonable until the phenomenon and conditions are explained.

By the frequency of a current is meant the rapidity of its alternations or reversals in direction. Joseph Henry, of Washington, first discovered that the discharge of a Leyden jar or condenser is oscillatory, and Sir Oliver Lodge was able to visibly analyze such a spark discharge by means of a rapidly rotating mirror. This gave him definite knowledge that each discharge, instead of being a single spark, was a quick succession of flashes in opposite directions, beginning with a heavy flash and rapidly dying down to zero. The discharge of an open induction coil will seldom exceed ten thousand alternations per second, while the discharge from a suitable condenser may range from a few hundreds of oscillations per second to several billion, depending on the capacity and inductance of the circuit.

These termsócapacity and inductance may be more readily understood if they are likened to the elasticity and inertia of a vibrating spring. In either case both qualities must be present in order that vibrations or oscillations shall occur. As a spring swings past its zero to be pulled back in the opposite direction, so does a Leyden jar over-discharge itself, then re-discharges in the opposite direction, and repeats this until the current falls away to zero. Such oscillations generate the familiar electromagnetic waves - of - wireless telegraphy, and because the oscillations rapidly die out so also do the waves, and we then say that they are strongly damped.

From the above, one can see in a simple manner how the wave-length depends upon capacity and inductance.

In order to create more permanent oscillations a source of constant and high potential may be shunted around the condenser, causing a continual process of charging and discharging. The first experiments along this line were performed by Dr. Hertz of the University of Bonn, who used merely an open induction coil. After their discoverer, they are known as Herts effects.

While operating a coil with an exceedingly rapid vibrator, he noticed that sparks were jumping from several metallic objects about the room. By means of his resonator shown in Fig. 1, Hertz was able to prove that certain waves were sent out from the sparking circuit which could be caught by another circuit of proper wave-length, in which they would set up a sympathetic current capable of giving sparks. The best position for resonance was readily found by adjusting the length of the rectangle "C," and by placing it in different positions, the plane and direction of the wave was determined. The two circuits responded to each other just as two tuning forks of like pitch will respond sympathetically.

Following up these trail-blazing experiments, Nikola Tesla achieved fame by a number of more striking ones. Tesla determined to produce high frequency currents direct from the dynamo, and with this in view he constructed a number of machines, the plan of one of which is shown in Fig. 2, which he called high frequency alternators. The coils are very small and the poles hardly more than fine teeth. With such a machine Tesla was able to obtain a frequency of 10,000 per second, and 10 amperes at 100 volts. The machine, small as it was, had 400 poles on the field, 400 coils on the armature and was run at 3,000 revolutions per minute. In a later machine a frequency of 15,000 per second was obtained. Since then others have labored in the same field, Duddell by using an induction disk with a fine toothed edge obtained at first a frequency of 50,000 per second and later the marvelous one of 120,000, although with the latter the current was reduced to .1 of an ampere and the voltage to 2 volts.

Professor Trowbridge, who was experimenting at the same time, produced sparks from 6 to 7 feet long from a condenser discharge which he calculated to be 3,000,000 volts! His method of obtaining such a voltage was simple but ingenious. Using 20,000 small storage cells, he charged the plates of a large condenser which were arranged in multiple. Reversing their arrangement he connected the plates in series and thus had an enormous potential though an exceedingly small current at his disposal.

However, Tie was outdone by Tesla, who by his well-known Tesla coil or secondary transformer, produced a roaring spark of sixteen feet in length in 1900. He virtually reproduced lightning itself by a deafening discharge that crashed between electrodes a hundred feet apart. The voltage was in the billions and the current about 800 amperes.

The secondary transformer or Tesla coil used by him is shown diagrammatically in Fig. 3, where it is readily seen that the current from the induction coil sets up oscillations in the secondary circuit through induction which are of both enormous voltage and very high frequency due to the capacity and inductance of the circuit.

The effect of such a spark on the body is peculiar. Where it strikes the body direct it may be more painful than the ordinary spark, but if made to strike an electrode of some metal touching the body or held in the hand, the whole discharge may be taken without any shock whatever. In such a situation sparks may be drawn from any part of the person and a gas jet may be lighted with the finger without even being insulated from the ground. To prove that an amount of current is really being conducted by the body, an incandescent lamp held in the hand, between it and the terminal, may be lighted.

Two probable reasons are advanced to explain the absence of sensation. In the first place an electric current starts to travel on the surface and spreads in a comparatively gradual manner to the interior of a, conductor. When the successive impulses of the current are very short it travels only on the surface and therefore it is called the skin effect. Secondly, the sensory nerves of the body do not respond to such a short stimulus as probably the millionth part of a second.

The above mentioned skin effect is responsible for another statement at the beginning of this article. The larger a conductor is, the less will the current penetrate beneath the surface, and consequently the greater will be the resistance. This is at first a seeming incompatability, but a mathematical investigation will show the exact rule, that the effective resistance to a high frequency current varies directly as the diameter of the conductor. Thus if a current is passed through a very heavy bar, the effective resistance of the bar will be so great as to make it possible to light a lamp by touching its terminals only a few inches apart upon the bar, although the lamp might seem to be virtually short-circuited.

Tesla labored long attempting to produce light direct for illuminating purposes without the necessity of cumbersome conductors.

He noticed five distinct forms of secondary discharge from his coil, controlled by a change in frequency or strength. Three were different forms of brush discharge; one a weak, sensitive, threadlike discharge, and one a powerful flaming discharge. Under certain conditions brush streamers issued from all parts of the coil, even through the insulation, while the brush in its hottest form resembled a jet of burning gas, giving off ozone freely. By stretching parallel wires across a room, a sheet of light may be caused to appear between them while an incandescent bulb or a vacuum tube would glow with a pale bluish luminescence if held near one pole of the coil.

The effect of such a current upon air or gas is to alternately attract and repel the particles, setting them in exceeding rapid vibration, thus a large amount of heat is generated and luminosity occurs. Something of this condition, it is suggested, may explain some of the phenomena of the Aurora, and if such effects could be regulated over a large area of air space, storms and rains could likely be controlled.

There are a great many Tesla effects of wondrous and striking interest and of much scientific importance, with reference to artificial illumination.

With a vacuum tube and a single conducting wire shown in Fig. 4, Tesla obtained a high degree of luminosity. If the filament inside of the tube was flexible it would often rotate in a circle as shown by the dotted lines due to its bombardment by air particles. Mr. Tesla himself believed that a new era of illumination would soon be entered upon, following the discovery that luminosity could be produced directly by the high frequency discharge, or indirectly, as in Fig. 5, with an unconnected tube within the influence of the electrostatic field.

Less progress has been made in this direction than might have been expected, though the Moore lamp is of such a type. It is a long tube made to fit the room, and taking about .3 ampere at 12,000 volts. The color of the light is affected by the kind of gas present in the tube, and daylight itself may be quite successfully rivalled. A low efficiency and complication of details detract from the popularity of the light, however.

The latest use for light frequency currents is the production of a wave suitable for wireless telephony, and one of the best present methods for producing these currents is by the musical arc discovered by Duddell. He shunted a condenser and inductance around an arc light circuit and found that the condenser rapidly charged and discharged itself through the arc, setting up oscillations of great regularity and persistence.

Such a simple arrangement gave a frequency of 10,000 per second. Valdemar Poulsen, by surrounding the arc with hydrogen and placing it in a magnetic field, obtained much higher frequencies up to 1,000,000 per second, due to the cooling effect of the hydrogen and the tendency of the magnetic field to blow the arc. The waves sent out by such a device are of such high frequency and constancy as to permit transmitting the human voice with all of its delicate inflections.

This alluring field of high frequency currents, is not like many attractions, gilt on the outside only, but is solid gold. What it holds for us in the future we do not know, for achievements of the past are only a foretaste.



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