Parasites of the Ether

Radio Record, Volume V, Issue 15, 23 October 1931, Unnumbered Page

Parasites of the Ether

The Causes of Atmospherics

HE reach of a modern multivalve set, using screened-grid amplification, is now limited solely by the electrical "uzivest" of the atmosphere. ‘The wider ‘the range, the greater ‘becomes the intake of at3 mospherics, "until the general: level of "noise" drowns out the more distant. signals. Luckily such disturbances are not so prevalent here as in; the tropics, where a continual succession of crashes, bangs and rattles often renders ordinary re-. ception quite impossible for hours at atime. In any locality, however, atmospherics are usually more troublesome on the long waves than on the short. They prefer the summer time to winter, and are subject to peculiar. variations at sunrise and sunset. _ If one commences to listen about half an hour before sunrise, the strays are loud and numerous-much as they have been all night. About fifteen minutes before sunrise they get weaker and fewer, rather: suddenly, until a lull sets in, which lasts for perhaps a minute. They then reappear iu force, and within ten minutes they . have settled down to a steady roar. 'ATMOSPHERICS can be classified as grinders, clicks, and hisses. "Clicks" are due to definite atmospheric discharges, more or less distant; while "hisses" are caused by an intermittent flow of current through the aerial, when it taps a point of high atmospheric potential. The term "grinders" coyers all the other miscellaneous noises. . To understand the origin of atmospheric disturbances one must regard Et

the earth as being surrounded by a sea of so-called "static" electricity. If it were really "static" in the sense of being "quiescent," all would be well. Actually, there are continual dynamic discharges taking place in the atmosphere, and these cause all the trouble. The potential gradient in the sea of static, moving upward from ground level, is approximately 100 volts per metre. This, of course, implies a corresponding charge of electricity on the surface of the earth. Interchanges take place between the earth charge and the surrounding sea of "static" whenever any change occurs in the preyailing meteorological conditions. DURING fine weather there is usually a downward positive current, tending to neutralise a positive charge on the surface of the earth, while during bad weather, currents may flow in both directions. . ‘When rain moisture, for instance, is first precipitated in tiny drops, the latter acquire the potential of the atmosphere at that particular spot. AS they form into a cloud the tiny drops coalesce into slightly larger drops, ‘and ‘the capacity of each diminishes. ‘However, since the original charge is .retained, the potential increases, and ‘is communicated to the cloud, which, in. this way, accumulates an excessive ‘charge,

Similarly, a fall of snow will cause a redistribution of atmospheric electricity. Or a current of warm or cold air may have the same effect. Sometimes. the resultant -potential differences between the surface of the earth and a charged cloud (or between one cloud and another) becomes so intense that the voltage breaks down the insulation, and a lightning. discharge takes place. . Broadly speaking, however, every factor which tends to upset the _ static equilibrium is‘ quickly followed by corresponding electrical movements tending to restore equilibrium. All such movements are liable to affect a sensitive wireless receiver and create "noise" in the set. It has been calculated that an average flash of lightning, two kilometres long, discharges a current of 20 coulombs in about the one five-hundredth part of a second-yrepresenting an average current of 10,000 amps. By .comparison a modern highpowered transmitter will feed, say, 500 amps. into an aerial 800 ft..long, and has an effective range of some 20,000 miles or more. _ he lightning fiash, while it lasts, is 200 times more powerful than the transmitter, so that there is little wonder that it makes a strong impression even — =

EEE when very far distant on a sensitive radio receiver. : ‘TaE static content of the earth’s atmosphere is largely influenced by the sun... It is calculated that the amount of solar radiation reaching the earth each day is equivalent to twenty thousand’ billion horsepower. ‘This energy is chiefly composed of electromagnetic waves, visible light, ultraviolet rays, and streams of electrons and ions. In addition the earth reeceives a Jiberal daily allowance of a very penetrating type of radiation (Millikin rays) from interstellar space. All this has a very definite effect in ionising the atmosphere and charging it with static. For instance, streams of ions and electrons projected from the sun, particularly: during a period of sunspot ‘activity, swirl around the earth’s magnetic field and create space currents. These in turn induce corresponding "ground" currents ing the interior of the earth, and so gird rise +; to magnetic disturbances which react ; upon the electrical equilibrium of the ' atmosphere. Regarding the earth as immersed in } a sea of static, the passage of deliber_ately transmitted wireless signals can be compared with a small but regular succession of wavelets. . Superimposed on.these»one must imagine a constant fret of atmospheric "disturbances" comprising irregular movements and surgings, such as are caused in the ocean by wind, tide, and other natural influences. Such disturbances may vary in size from infinitesimal ripples to full-sized waves, and even an occasional tidal flood. -P, 0. Jevons, in "Popular Wireless.™: yy