Page:EB1911 - Volume 02.djvu/913

 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.

.—Dissipation. Mean Values. 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 and 1, 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.

.—Dissipation.

15. Diurnal Variation.—P. B. Zölss (41, 42) has published diurnal variation data for Kremsmünster for more than one year, and independently for midsummer (May to August) and midwinter (December to February). His figures show a double daily period in both a+ and a−, the principal maximum occurring about 1 or 2 The two minima occur, the one from 5 to 7, the other from 7 to 8 ; they are nearly equal. Taking the figures answering to the whole year, May 1903 to 1904, a+ varied throughout the day from 0·82 to 1·35, and a− from 0·85 to 1·47. At midsummer the extreme hourly values were 0·91 and 1·45 for a+, 0·94 and 1·60 for a−. The corresponding figures at midwinter were 0·65 and 1·19 for a+, 0·61 and 1·43 for a−. Zölss’ data for q show also a double daily period, but the apparent range is small, and the hourly variation is somewhat irregular. At Karasjok, Simpson found a+ and a− both larger between noon and 1 than between either 8 and 9  or 6 and 7  The 6 to 7  values were in general the smallest, especially in the case of a+; the evening value for q on the average exceeded the values from the two earlier hours by some 7%.

Summer observations on mountains have shown diurnal variations very large and fairly regular, but widely different from those observed at lower levels. On the Rothhorn, Gockel (43) found a+ particularly variable, the mean 7 value being 4 times that at 1  q (taken as Σ(a−/a+) varied from 2·25 at 5  and 2·52 at 9  to 7·82 at 3  and 8·35 at 7  On the Sonnblick, in early September, V. Conrad (22) found somewhat similar results for q, the principal maximum occurring at 1, with minima at 9  and 6 ; the largest hourly value was, however, scarcely double the least. Conrad found a− largest at 4  and least at 6 , the largest value being double the least; a+ was largest at 5  and least at 2 , the largest value being fully 2 times the least. On Mont Blanc, le Cadet (43) found q largest from 1 to 3 , the value at either of these hours being more than double that at 11  On the Patscherkofel, H. von Ficker and A. Defant (52), observing in December, found q largest from 1 to 2  and least between 11  and noon, but the largest value was only 1 times the least. On mountains much seems to depend on whether there are rising or falling air currents, and results from a single season may not be fairly representative.

16. Dissipation seems largely dependent on meteorological conditions, but the phenomena at different stations vary so much as to suggest that the connexion is largely indirect. At most stations a+ and a− both increase markedly as wind velocity rises. From the observations at Trieste in 1902–1903 E. Mazelle (47) deduced an increase of about 3% in a+ for a rise of 1 km. per hour in wind velocity. The following are some of his figures, the velocity v being in kilometres per hour:—

For velocities from 0 to 24 km. per hour q exceeded unity in 74 cases out of 100; but for velocities over 50 km. per hour q exceeded unity