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579. The identity of lightning and electricity was first suspected by Wall in 1708, but it was reserved to Franklin to prove it. In 1749, he suggested, as the mode of proof, the erection of pointed metallic conductors properly insulated. Acting on this suggestion, Dalibard erected near Paris a pointed iron rod, 40 feet in length, and insulated ; and on the 10th of May 1752 obtained electrical sparks from it. In June of the same year, Franklin, impatient at the delay in erecting the spire on which to place his pointed conductor, conceived the happy idea of obtaining electricity from the clouds by flying a kite. The kite was flown with a hempen string, to the lower end of which a key was attached ; and the whole was insulated by tying a silk ribbon to the key, the other end of the ribbon being attached to a post. Or the approach of the thunder-cloud, he raised the kite, anà soon the fibres of the hempen string began to erect themselves and repel each other; and at last, when the rain had moistened the string, he had the intense satisfaction of drawing electrical sparks from the key. The experiment was repeated by Romas, during a thunderstorm in France, in June 1753. Instead of a string, he used fine wire (550 feet long), and obtained flashes of electrical fire, 9 or 10 feet long, and an inch in thickness, which were accompanied with a loud report. Thirty of these were obtained in one hour. In August of the same year, Professor Richmann of St Petersburg lost his life when engaged in similar experiments. In observing the electricity of clouds when the tension is great, a metallic ball must be placed near the bar, and be well connected with the ground; and care must be taken to keep at a greater distance from the bar than the ball,'so that if a discharge should take place it will strike the ball and not the observer.
580. Electrometers are instruments used for indicating the electricity present in the atmosphere. A pole is erected in an open situation on a rising ground, having an insulated pointed metallic wire on the top, to which an insulated wire is attached for conveying the electricity to the electrometer in the place of observation. Fig. 47 represents Bennet's gold-leaf electrometer, which consists of a glass jar with a metallic cap, in the centre of which a wooden wedge is inserted. On each side of it a thin strip of gold-leaf, two inches long, is attached, and opposite each, tinfoil is pasted within the jar, rising a little above the lower edge of the gold-leaf, and
Fig. 49. connected below with the brass stand of the instrument. A pointed wire rests on the cap in connection with the gold-leaf. When this pointed wire receives electricity, the gold-leaves diverge, and by the degree of divergence, measured on a graduated arc, the intensity of the electricity is ascertained. A condenser is used when the electric tension is too feeble to cause the goldleaves to diverge. In Volta's electrometer, two thin blades of straw are used instead of the gold-leaf; and in Cavallo's, two pith-balls. In Henley's quadrant electrometer, fig. 48, a semicircle of ivory is fixed upon a rod rising from a stand, from the centre of which a pithball is hung by a piece of slender cane; and Fig. 50. the degree of elevation of this ball indicates the quantity of the electricity.
581. Electroscopes show whether the electricity is positive or negative. In Bohnenberger's electroscope, a gold-leaf is suspended between the poles of two “dry piles," charged with the opposite electricities; when, therefore, an electric body is brought into contact with the knob, the kind of electricity is known by the gold-leaf being attached to the opposite electricity.
582. It has been found that the atmosphere always contains free electricity, which is almost invariably positive. At Kew Observatory, during the years 1845-6-7, of the 10,500 observations, 10,176 were positive, and only 364 negative. When the sky is cloudless, the electricity is always positive; but the intensity varies with the height, being greatest in the highest and most isolated situations. Positive electricity is only found at a certain height above the ground. On flat ground it becomes manifest at a height of 5 feet. It is not found in houses, in streets, or under trees. The negative observations almost all occurred during heavy rain. When the sky is clouded, the electricity is sometimes positive and sometimes negative, according to the electrified condition of the clouds. In relation to the air the earth's surface is always negative.
583. The electricity of the air increases in intensity with the height. This was shown by an ingenious experiment made at the Great St Bernard, by Becquerel and Breschet. A silk cord, with a fine wire twisted into it, was attached to an electrometer at one end, and an iron arrow tied to the other, and was shot from a bow to the height of 250 feet. As the arrow ascended, the thin straws of the electrometer separated more and more, and at last struck against the sides of the jar. The arrow was then shot horizontally, but no increase of the electric tension was observed. This conclusion has been confirmed by flying paper kites, and sending up captive balloons into the air.
584. The electricity of the atmosphere is stronger in winter than in summer, increasing from June to January, and decreasing from January to June. It is subject to a double maximum and minimum each day. Saussure and Schübler have shown that the first maximum occurs from 7 to 8 A.M. in summer, and from 10 A.m. to noon in winter ; it then falls slightly to the first minimum between 5 and 6 P.M. in summer, and between 2 and 3 P.m. in winter, or shortly after the period of the maximum temperature ; it rises to a second maximum a little after sunset, and then decreases to a second minimum, which occurs about daybreak. The daily march of the electric tension is best marked in clear settled weather.
585. Sources of Atmospheric Electricity.—1. Evaporation. -Electricity is produced when impure water is evaporating, or water in which some degree of chemical decomposition takes place, none whatever being produced by the evaporation of pure water. Evaporation from water containing an alkali or a salt gives off negative electricity to the air, and leaves positive electricity behind ; but when the water contains acid, positive electricity is given off, and negative is left behind. Hence it is supposed that seas, lakes, and rivers are abundant sources of electricity, particularly of the positive sort. 2. Vegetation.—The vegetable kingdom is also a source of electricity, (1) from the evaporation going on by which water is separated from the sap of the plants, and (2) from the giving off of oxygen gas during the day, and carbonic gas during the night. In these cases positive electricity arises from the plants, and negative is left behind. 3. Combustion.
-During the process of burning, bodies give off positive electricity, and become themselves negatively electrified. This is frequently seen on a grand scale during volcanic eruptions. 4. Friction.—Wind, by the friction it produces upon terrestrial objects, the particles of dust, and the watery particles in the vesicular state which it carries with it, contributes to the electricity of the air. Electricity is not generated, if the moisture be in the form of pure vapour.
586. Eject of the Condensation of Vapour.—The following are Sir John Herschel's views on the relations subsisting between the condensation of vapour and atmospheric electricity. When a great multitude of the ultimate molecules of vapour are condensed by cold into a drop or snow-spangle, however minute, that drop collects and retains on its surface the whole electricity of the molecules from which it is formed. Suppose a number, say 1000, of such globules to coalesce into one, the electric contents, being the sum of those of the elementary globules, will be increased one thousandfold, and, being spread entirely over the surface, will have a tenfold density or tension. This view appears to explain the amount of electricity observed in the lower stratum of air when dew is being deposited, and the highly electrical state of fogs and clouds. It also explains the annual fluctuation ; for since in winter the condensation of vapour is greater and occurs with greater frequency than in summer, the average quantity of electricity, as observed; will be greater in winter than in summer. At this season, also, the south-west winds acquire their greatest frequency. The daily fluctuation is similarly explained. When the sun has risen, and from the increasing temperature evaporation is most active, the first maximum period of electricity occurs; but when the air becoming still warmer is able to hold its vapour in solution, and evaporation has become feebler, the intensity decreases. Again, after sunset, when dew is most copiously deposited, the second maximum period is attained; and lastly, toward sunrise, when little dew is deposited, and evaporation is also small, the second and chief minimum period occurs. But great differences of opinion exist in regard to the electricity of the atmosphere, and many of the phenomena, especially those observed during storms, have not yet been explained. A sufficient number of trustworthy observations are the great desideratum ; and not till some instrument has been devised of such a description that the observations made with it in different places may be comparable with each other, and the price be at the same time no barrier to its general use as a meteorological instrument, can we hope to be in a position adequately to investigate the subject.