12. Elster and Geitel (35) have measured the charge carried by raindrops falling into an insulated vessel. Owing to observational difficulties, the exact measure of success attained is a little difficult to gauge, but it seems fairly certain that raindrops usually carry a charge. Elster and Geitel found the sign of the charge often fluctuate repeatedly during a single rain storm, but it seemed more often than not opposite to that of the simultaneous potential gradient. Gerdien has more recently repeated the experiments, employing an apparatus devised by him for the purpose. It has been found by C.T.R. Wilson (36) that a vessel in which freshly fallen rain or snow has been evaporated to dryness shows radioactive properties lasting for a few hours. The results obtained from equal weights of rain and snow seem of the same order.
13. W. Linss (6) found that an insulated conductor charged either positively or negatively lost its charge in the free atmosphere; the potential V after time t being connected with its initial value V0 by a formula of the type V = V0e−at where a is constant. This was confirmed by Elster and Geitel (7), whose form of dissipation apparatus has been employed in most recent work. The percentage of the charge which is dissipated per minute is usually denoted by a+ or a− according to its sign. The mean of a+ and a− is usually denoted by a± or simply by a, while q is employed for the ratio a−/a+. Some observers when giving mean values take Σ(a−/a+) as the mean value of q, while others take Σ(a−)/Σ(a+). The Elster and Geitel apparatus is furnished with a cover, serving to protect the dissipator from the direct action of rain, wind or sunlight. It is usual to observe with this cover on, but some observers, e.g. A. Gockel, have made long series of observations without it. The loss of charge is due to more than one cause, and it is difficult to attribute an absolutely definite meaning even to results obtained with the cover on. Gockel (37) says that the results he obtained without the cover when divided by 3 are fairly comparable with those obtained under the usual conditions; but the appropriate divisor must vary to some extent with the climatic conditions. Thus results obtained for a+ or a− without the cover are of doubtful value for purposes of comparison with those found elsewhere with it on. In the case of q the uncertainty is much less.
Table VI.—Dissipation. Mean Values.
| Place. | Period. | Season. | Observer or Authority. | a+ | q |
| Karasjok | 1903-4 | Year | Simpson (10) | 3.57 | 1.15 |
| Wolfenbüttel | Year | Elster and Geitel (39) | 1.33 | 1.05 | |
| Potsdam | 1904 | Year | Lüdeling (40) | 1.13 | 1.33 |
| Kremsmüster | 1902 | Year | Zölss (42) | 1.32 | 1.18 |
| ” | 1903 | Year | Zölss (41) | 1.35 | 1.14 |
| Freiburg | Year | Gockel (43) | .. | 1.41 | |
| Innsbruck | 1902 | Czermak (44) | 1.95 | 0.94 | |
| ” | 1905 | Jan. to June | Defant (45) | 1.47 | 1.17 |
| Mattsee (Salzburg) | 1905 | July to Sept. | von Schweidler (46) | .. | 0.99 |
| Seewalchen | 1904 | July to Sept. | von Schweidler (38) | .. | 1.18 |
| Trieste | 1902-3 | Year | Mazelle (47) | 0.58 | 1.09 |
| Misdroy | 1902 | Lüdeling (40) | 1.09 | 1.58 | |
| Swinemünde | 1904 | Aug. and Sept. | Lüdeling (40) | 1.23 | 1.37 |
| Heligoland (sands) | 1903 | Summer | Elster and Geitel (40) | 1.14 | 1.71 |
| Heligoland plateau | ” | ” | Elster and Geitel (40) | 3.07 | 1.50 |
| Juist (Island) | ” | Elster and Geitel (48) | 1.56 | 1.56 | |
| Atlantic and German Ocean | 1904 | August | Boltzmann (49) | 1.83 | 2.69 |
| Arosa (1800 m.) | 1903 | Feb. to April | Saake (50) | 1.79 | 1.22 |
| Rothhorn (2300 m.) | 1903 | September | Gockel (43) | .. | 5.31 |
| Sonnblick (3100 m.) | 1903 | September | Conrad (22) | .. | 1.75 |
| Mont Blanc (4810 m.) | 1902 | September | le Cadet (43) | .. | 10.3 |
Table VI. gives the mean values of a± and q found at various places. The observations were usually confined to a few hours of the day, very commonly between 11 A.M. and 1 P.M., and in absence of information as to the diurnal variation it is impossible to say how much this influences the results. The first eight stations lie inland; that at Seewalchen (38) was, however, adjacent to a large lake. The next five stations are on the coast or on islands. The final four are at high levels. In the cases where the observations were confined to a few months the representative nature of the results is more doubtful.
On mountain summits q tends to be large, i.e. a negative charge is lost much faster than a positive charge. Apparently q has also a tendency to be large near the sea, but this phenomenon is not seen at Trieste. An exactly opposite phenomenon, it may be remarked, is seen near waterfalls, q becoming very small. Only Innsbruck and Mattsee give a mean value of q less than unity. Also, as later observations at Innsbruck give more normal values for q, some doubt may be felt as to the earlier observations there. The result for Mattsee seems less open to doubt, for the observer, von Schweidler, had obtained a normal value for q during the previous year at Seewalchen. Whilst the average q in at least the great majority of stations exceeds unity, individual observations making q less than unity are not rare. Thus in 1902 (51) the percentage of cases in which q fell short of 1 was 30 at Trieste, 33 at Vienna, and 35 at Kremsmünster; at Innsbruck q was less than 1 on 58 days out of 98.
In a long series of observations, individual values of q show usually a wide range. Thus during observations extending over more than a year, q varied from 0.18 to 8.25 at Kremsmünster and from 0.11 to 3.00 at Trieste. The values of a+, a− and a± also show large variations. Thus at Trieste a+ varied from 0.12 to 4.07, and a− from 0.11 to 3.87; at Vienna a+ varied from 0.32 to 7.10, and a− from 0.78 to 5.42; at Kremsmünster a± varied from 0.14 to 5.83.
14. Annual Variation.—When observations are made at irregular hours, or at only one or two fixed hours, it is doubtful how representative they are. Results obtained at noon, for example, probably differ more from the mean value for the 24 hours at one season than at another. Most dissipation results are exposed to considerable uncertainty on these grounds. Also it requires a long series of years to give thoroughly representative results for any element, and few stations possess more than a year or two’s dissipation data. Table VII. gives comparative results for winter (October to March) and summer at a few stations, the value for the season being the arithmetic mean from the individual months composing it. At Karasjok (10), Simpson observed thrice a day; the summer value there is nearly double the winter both for a+ and a−. The Kremsmünster (42) figures show a smaller but still distinct excess in the summer values. At Trieste (47), Mazelle’s data from all days of the year show no decided seasonal change in a+ or a−; but when days on which the wind was high are excluded the summer value is decidedly the higher. At Freiburg (43), q seems decidedly larger in winter than in summer; at Karasjok and Trieste the seasonal effect in q seems small and uncertain.
Table VII.—Dissipation.
| Place | Winter | Summer | ||||||
| a+ | a− | a± | q | a+ | a− | a± | q | |
| Karasjok 1903-1904 | 2.28 | 2.69 | 2.49 | 1.18 | 4.38 | 4.94 | 4.65 | 1.13 |
| Kremsmüster 1903 | 1.14 | 1.30 | 1.22 | 1.14 | 1.38 | 1.56 | 1.47 | 1.12 |
| Freiburg | .. | .. | .. | 1.57 | .. | .. | .. | 1.26 |
| Trieste 1902-1903 | 0.56 | 0.59 | 0.58 | 1.07 | 0.55 | 0.61 | 0.58 | 1.13 |
| Trieste calm days | .. | .. | 0.35 | .. | .. | .. | 0.48 | .. |