806 



ELECTRICITY, ATMOSPHEEIC. 



ELECTRICITY, ATMOSPHERIC. 



also, the intensity of the atmospherical electricity is generally dimi- 

 nished, probably because the strata of air containing different quanti- 

 ties of the fluid are brought successively to the ground, and thus there is 

 produced a nearly uniform distribution of the fluid between the earth 

 and the atmosphere. It may be easily conceived that, in stormy weather, 

 the variations of the atmospherical electricity will be very irregular ; 

 for currents of air in the upper regions, driving the strata of clouds in 

 different directions, the electrical actions between the clouds and the 

 atmosphere below must be extremely complex. 



M. de Saussure has observed that, during summer aud winter, by 

 night as well as by day, when the atmosphere is free from clouds, the 

 electricity of the air is positive ; and Mr. Read (' Phfl. Trans.,' 1794) 

 has shown that out of 404 observations made hi one year, the air was 

 positively electrical in 241, negatively in 156, and that the electricity 

 was insensible in 7 observations only. It seems probable, hi fact, that 

 the negative electricity which may be observed in a pure atmosphere is 

 caused by the discharge of its positive electricity into the earth or a 

 cloud, when one or the other of these is in a contrary state. Mr. 

 Ronayne, however, states ('Phil. Trans.,' 1772) that in Ireland the 

 electricity of the atmosphere is positive hi whiter when the air is clear : 

 he observes that it diminishes in frosty or foggy weather, and that he 

 could detect no electricity in the air during the summer except when 

 a fog came on ; the electricity was then positive, but it was less intense 

 than during the winter fogs. It was an observation of Saussure that 

 electricity is strongest in the open air, and that it is weak in streets, in 

 houses, and under trees. In close rooms and hospitals the electricity 

 of the air hag been always found to be negative ; such also is the elec- 

 tricity of the atmosphere when it is vitiated by exhalations from lime, 

 paint, and decaying vegetables. [OZONE.] 



All observations concur in showing diurnal variations in the intensity 

 of atmospherical electricity, but there is some uncertainty concerning 

 the precise times at which the intensities are the greatest and the least. 

 Cavallo observed that in dry weather the electricity was weakest at 

 sunrise ; that it attained the maximum of strength in the day-time, 

 and continued in that state till sunset, when the intensity diminished. 

 He conceived that this diminution was much more rapid as the air 

 became more humid ; and he observed that in whiter, when a dry wind 

 prevailed, if the sky were free from clouds, the electricity became very 

 strong after sunset. M. de Saussure observed at Geneva (1785) that, 

 during whiter, the intensity of atmospherical electricity attained its 

 first maximum at 9 A.M. ; that it diminished till 6 P.M., when it was in 

 a minimum state ; it afterwards increased, aud attained its second 

 maximum at 8 P.M. ; after which it continued to dimmish till it was 

 again a minimum at 6 on the following morning. The same philoso- 

 pher found that in summer the diurnal variations were less perceptible : 

 on a dry warm day he found the electricity increase from sunrise, when 

 it was almost insensible, till 3 or 4 P.M., when it became a maximum ; 

 it appeared then to diminish till the dew fell, when it became stronger, 

 but it was scarely sensible during the night. Lastly, the experiments 

 of Mr. Read exhibit two maxima and two minima hi twenty-four hours. 

 The atmospherical electricity seemed strongest at two or three hours 

 after sunrise, and again about sunset ; it was weakest at noon and at 

 4 P.M. 



The experiments of Mr. Crosae show also that, hi the ordinary state 

 of the atmosphere, the electricity is positive, and that it increases in 

 proportion to the elevation above the earth's surface ; the same phi- 

 losopher observes that it is most intense at sunrise and sunset, and 

 weakest at noon and during the night. He finds that the approach of 

 a thundercloud produces a change in the electricity of the atmosphere, 

 rendering it positive if it were before negative ; and the contrary : 

 whatever be the nature of the change which takes place, the intensity 

 of the electricity increases to a certain degree ; it then diminishes and 

 disappears, and is succeeded by an opposite electricity : this gradually 

 increases till it becomes of higher intensity than the former kind, and 

 then it decreases till it vanishes : it is again succeeded by the first 

 kind. These changes are often found to take place several tunes 

 successively. Fogs, rain, snow, &c., also change the electricity from 

 positive to negative, again from negative to positive, and so on ; the 

 change taking place every three or four minutes. A cold rain, in large 

 drops, is frequently accompanied by intense electricity ; and during a 

 driving fog or rain the electricity is occasionally as strong as during a 

 thunder-storm. A warm small ruin is weakly electrified ; and a weak 

 positive electricity generally prevails during cloudy weather. Mr. 

 Croe finds also that the electricity of the air is very weak during the 

 north-east winds which in winter and spring tunes produce extreme 

 cold and dryness. 



The intensity of atmospherical electricity has been observed to 

 undergo annual changes ; it increases from July to November inclu- 

 nive ; so that the greatest intensity occurs in winter, and the least hi 

 Hummer. 



Any of the different kinds of electrometer may be employed to 

 determine the nature and intensity of atmospherical electricity. 

 [ ELF.CTBOMETER.] Professor Loomis states ( Poggendorff's Aunalen, 

 Band 100), that the telegraphic wires are very sensitive to an approach- 

 ing thunder-storm, and often become highly charged even when a 

 Htonn is so distant that neither thunder nor lightning can be appre- 

 ciated; but if the thunder-cloud be near, the wires may become so 

 highly charged M to injure the magnetic apparatus and expose the 



clerks to considerable danger. Such effects, however, aro not peculiar 

 to America. 



Later observations have confirmed the conclusion that the usual 

 electrical condition of the air is positive. Out of 15,170 observations 

 made at the Kew Observatory, during a period of 5 years, 14,515 are 

 of positive electricity, and 655 of negative. During the years 1845, 

 1846, and 1847, out of 10,500 observations, 10,176 showed positive, 

 and only 324 negative electricity ; the latter being usually accompanied 

 by heavy ram. The tension of atmospheric electricity was found to 

 be at its minimum at 2 a.m., from which hour there was a gradual 

 increase until 6 a.m. ; after which the tension increased more rapidly, 

 ita value at 8 a.m. being nearly double that at 6 a.m.: the increase 

 then became more gradual until 10 a.m., the period of the first or 

 morning maximum : from this hour it gradually declined until 4 p.m., 

 when it was only a little higher than at 8 a.m. ; it then increased 

 rapidly until 8 p.m., and after a slight rise at 10 p.m., the time of the 

 principal or evening maximum, the tension was found no longer to 

 increase. The evening maximum was much higher than that of the 

 morning. Between 10 p.m. and midnight the tension decreased nearly 

 to that of the diurnal minimum. With regard to the annual period 

 the lowest tensions were found to be in June and August, the tension 

 during July being slightly superior to that of those two months. 

 There is a slight elevation hi September, which increases in October, 

 and more rapidly from November to January. The maximum is in 

 February. In March a rapid diminution sets in, and proceeds to the 

 minimum in June. 



During four years' observations made at Brussels by M. Quetelet, 

 the maximum was found to be in January, and the minimum in June. 

 Expressed numerically the maximum had the value of 605, and the 

 minimum 47; so that the electricity in January is thirteen times more 

 energetic than in June. The difference was found to be much more 

 sensible during serene than cloudy weather, and, setting out from 

 June, the electricity of the air in serene weather was found to exceed 

 the electricity observed during a clouded sky, in proportion as January 

 was approached, hi which month the ratio was more than 4 to 1. The 

 powerful electric intensity of the air during a serene sky in winter 

 must be considered as a remarkable fact. A high electric intensity 

 was always observed during fog and snow. 



Atmospheric electricity seems to depend for its supply chiefly on 

 evaporation. It can be proved by experiment that the evaporation of 

 water from heated bodies disturbs the electrical equilibrium, the + 

 electricity being carried off by the vapour, while the electricity is left 

 behind in the vessel, or is carried off by its support. In order to pro- 

 duce this result, however, some chemical change seems to be necessary, 

 since the evaporation of pure water fails to produce electrical dis- 

 turbance. When water is evaporated from alkaline solutions the 

 vapour carries off and leaves + electricity behind ; but the reverse 

 of this takes place when water is evaporated from an acid or from 

 neutral saline solutions, including sea-salt. During the processes of 

 vegetation, in which water is largely separated from the other con- 

 stituents of plants, there is also electrical disturbance. Volcanic 

 eruptions and conflagrations are also accompanied by strong electrical 

 excitement. The friction of wind, dust, &c., is also a source of atmos- 

 pheric electricity. Dr. Livingstone (' Missionary Travels, &c., in South 

 Africa,' 1857) states that in the Kalahari Desert, during the dry 

 season, a hot wind occasionally blows, and it is " in such an electric 

 state that a bunch of ostrich feathers held a few seconds against it, 

 becomes as highly charged as if attached to a powerful electrical 

 machine, and clasps the advancing hand with a sharp crackling sound." 

 Even the motion of a native hi his kaross is sufficient to produce 

 therein a stream of small sparks. 



There are many difficulties in accounting for the presence of free 

 electricity in the air. Perhaps the best theory is that started by Bales 

 (' Phil. Trans.' 1757-8), and extended by Sir John Herschel (' Encyc. 

 Brit.,' 8th ed., Art. Meteorology). We will state sufficient to indicate 

 the nature of the theory. When water evaporates it is supposed to 

 pass off in the form of little vesicles of moisture, each vesicle charged 

 with electricity of too low a tension to discharge itself into neighbour- 

 ing vesicles, or to escape by conduction through a moist atmosphere. 

 If, however, a tall pointed conductor, a flame, or a column of smoke, 

 be introduced, the air may give off a portion of electricity by contact 

 discharge. But if we suppose a number of the particles of vapour 

 thus electrified to be condensed by cold into a drop of water, the whole 

 of the electricity would be collected on the surface of such drop with 

 a corresponding increase in tension. Suppose one such drop to be 

 1000 tunes the size of one of its constituent particles, the diameter 

 will be increased 10 tunes and the surface 100 times, while the elec- 

 tricity being the sum of the 1000 globules will be increased 1000 times 

 on the surface of the drop with a tenfold density or tension. The 

 comparatively high electric state of fog, and of snow, is thus ex- 

 plained. Every globule of moisture of which it consists has a coating 

 of electricity, which it will part with to the surface of any conductor, 

 and the denser the fog, and the larger its globules, the greater is the 

 quantity of electricity set free. This theory also explains the elec- 

 tricity of the air as occasioned by the deposit of dew, and also the 

 daily variation in tension ; for the air losing a great deal of vapour by 

 a deposit of dew, by night the electricity ia at its minimum. It 

 increases as fresh vapour arises under the influence of the ascending 



