+--------------------+---------------------------+---------------------------+
| | Winter. | Summer. |
+--------------------+------+------+------+------+------+------+------+------+
| Place. | 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 |
| Kremsmuster 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 |
| " calm days | .. | .. | 0.35 | .. | .. | .. | 0.48 | .. |
+--------------------+------+------+------+------+------+------+------+------+
15. _Diurnal Variation._--P.B. Zolss (41, 42) has published diurnal
variation data for Kremsmunster 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 P.M. The two minima
occur, the one from 5 to 7 A.M., the other from 7 to 8 P.M.; 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-. Zolss' 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 P.M. than
between either 8 and 9 A.M. or 6 and 7 P.M. The 6 to 7 P.M. 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 A.M. value being 4-1/2 times that at 1 P.M. q
(taken as [Sigma](a-/a+)) varied from 2.25 at 5 A.M. and 2.52 at 9
P.M. to 7.82 at 3 P.M. and 8.35 at 7 P.M. On the Sonnblick, in early
September, V. Conrad (22) found somewhat similar results for q, the
principal maximum occurring at 1 P.M., with minima at 9 P.M. and 6
A.M.; the largest hourly value was, however, scarcely double the
least. Conrad found a- largest at 4 A.M. and least at 6 P.M., the
largest value being double the least; a+ was largest at 5 A.M. and
least at 2 P.M., the largest value being fully 2-1/2 times the least.
On Mont Blanc, le Cadet (43) found q largest from 1 to 3 P.M., the
value at either of these hours being more than double that at 11 A.M.
On the Patscherkofel, H. von Ficker and A. Defant (52), observing in
December, found q largest from 1 to 2 P.M. and least between 11 A.M.
and noon, but the largest value was only 1-1/2 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:--
+-----+---------+-----------+-----------+-----------+
| v | 0 to 4. | 20 to 24. | 40 to 49. | 60 to 69. |
+-----+---------+-----------+-----------+-----------+
| a | 0.33 | 0.64 | 1.03 | 1.38 |
| q | 1.13 | 1.19 | 1.00 | 0.96 |
+-----+---------+-----------+-----------+-----------+
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
in only 40 cases out of 100. Simpson got similar results at Karasjok;
the rise in a+ and a- with increased wind velocity seemed, however,
larger in winter than in summer. Simpson observed a fall in q for wind
velocities exceeding 2 on Beaufort's scale. On the top of the
Sonnblick, Conrad observed a _slight_ increase of a[+-] as the wind
velocity increased up to 20 km. per hour, but for greater velocities
up to 80 km. per hour no further decided rise was observed.
At Karasjok, treating summer and winter independently, Simpson (10)
found a+ and a- both increase in a nearly linear relation with
temperature, from below -20 deg. to +15 deg. C. For example, when the
temperature was below -20 deg. mean values were 0.76 for a+ and 0.91
for a-; for temperatures between -10 deg. and -5 deg. the
corresponding means were 2.45 and 2.82; while for temperatures between
+10 deg. and +15 deg. they were 4.68 and 5.23. Simpson found no
certain temperature effect on the value of q. At Trieste, from 470
days when the wind velocity did not exceed 20 km. per hour, Mazelle
(47) found somewhat analogous results for temperatures from 0 deg. to
30 deg. C.; a-, however, increased faster than a+, i.e. q increased
with temperature. When he considered all days irrespective of wind
velocity, Mazelle found the influence of temperature obliterated. On
the Sonnblick, Conrad (22) found a[+-] increase appreciably as
temperature rose up to 4 deg. or 5 deg. C.; but at higher temperatures
a decrease set in.
Observations on the Sonnblick agree with those at low-level stations
in showing a diminution of dissipation with increase of relative
humidity. The decrease is most marked as saturation approaches. At
Trieste, for example, for relative humidities between 90 and 100 the
mean a[+-] was less than half that for relative humidities under 40. With
certain dry winds, notably Fohn winds in Austria and Switzerland,
dissipation becomes very high. Thus at Innsbruck Defant (45) found the
mean dissipation on days of Fohn fully thrice that on days without
Fohn. The increase was largest for a+, there being a fall of about 15%
in q. In general, a+ and a- both tend to be less on cloudy than on
bright days. At Kiel (53) and Trieste the average value of q is
considerably less for wholly overcast days than for bright days. At
several stations enjoying a wide prospect the dissipation has been
observed to be specially high on days of great visibility when distant
mountains can be recognize to be low on days of fog or rain.
The results obtained as to the relation between dissipation and
barometric pressure are conflicting. At Kremsmunster, Zolss (42) found
dissipation vary with the absolute height of the barometer, a[+-]
having a mean value of 1.36 when pressure was below the normal, as
against 1.20 on days when pressure was above the normal. He also found
a[+-] on the average about 10% larger when pressure was falling than
when it was rising. On the Sonnblick, Conrad (22) found dissipation
increase decidedly as the absolute barometric pressure was larger, and
he found no difference between days of rising and falling barometer.
At Trieste, Mazelle (47) found no certain connexion with absolute
barometric pressure. Dissipation was above the average when cyclonic
conditions prevailed, but this seemed simply a consequence of the
increased wind velocity. At Mattsee, E.R. von Schweidler (46) found no
connexion between absolute barometric pressure and dissipation, also
days of rising and falling pressure gave the same mean. At Kiel, K.
Kaehler (53) found a+ and a- both greater with rising than with
falling barometer.
V. Conrad and M. Topolansky (54) have found a marked connexion at
Vienna between dissipation and ozone. Regular observations were made
of both elements. Days were grouped according to the intensity of
colouring of ozone papers, 0 representing no visible effect, and 14
the darkest colour reached. The mean values of _a+_ and _a-_ answering
to 12 and 13 on the ozone scale were both about double the
corresponding values answering to 0 and 1 on that scale.
17. A charged body in air loses its charge in more than one way. The
air, as is now known, has always present in it ions, some carrying a
positive and others a negative charge, and those having the opposite
sign to the charged body are attracted and tend to discharge it. The
rate of loss of charge is thus largely dependent on the extent to
which ions are present in the surrounding air. It depends, however, in
addition on the natural mobility of the ions, and also on the
opportunities for convection. Of late years many observations have
been made of the ionic charges in air. The best-known apparatus for
the purpose is that devised by Ebert. A cylinder condenser has its
inner surface insulated and charged to a high positive or negative
potential. Air is drawn by an aspirator between the surfaces, and the
ions having the opposite sign to the inner cylinder are deposited on
it. The charge given up to the inner cylinder is known from its loss
of potential. The volume of air from which the ions have been
extracted being known, a measure is obtained of the total charge on
the ions, whether positive or negative. The conditions must, of
course, be such as to secure that no ions shall escape, otherwise
there is an underestimate. I+ is used to denote the charge on positive
ions, I- that on negative ions. The unit to which they are ordinarily
referred is 1 electrostatic unit of electricity per cubic metre of
air. For the ratio of the mean value of I+ to the mean value of I-,
the letter Q is employed by Gockel (55), who has made an unusually
complete study of ionic charges at Freiburg. Numerous observations
were also made by Simpson (10)--thrice a day--at Karasjok, and von
Schweidler has made a good many observations about 3 P.M. at Mattsee
(46) in 1905, and Seewalchen (38) in 1904. These will suffice to give
a general idea of the mean values met with.
+------------+-----------------+------+------+------+
| Station. | Authority. | I+ | I- | Q |
+------------+-----------------+------+------+------+
| Freiburg | Gockel | 0.34 | 0.24 | 1.41 |
| Karasjok | Simpson | 0.38 | 0.33 | 1.17 |
| Mattsee | von Schweidler | 0.35 | 0.29 | 1.19 |
| Seewalchen | " | 0.45 | 0.38 | 1.17 |
+------------+-----------------+------+------+------+
Gockel's mean values of I+ and Q would be reduced to 0.31 and 1.38
respectively if his values for July--which appear abnormal--were
omitted. I+ and I- both show a considerable range of values, even at
the same place during the same season of the year. Thus at Seewalchen
in the course of a month's observations at 3 P.M., I+ varied from 0.31
to 0.67, and I- from 0.17 to 0.67.
There seems a fairly well marked annual variation in ionic contents,
as the following figures will show. Summer and winter represent each
six months and the results are arithmetic means of the monthly values.
+--------+--------------------+--------------------+
| | Freiburg. | Karasjok. |
+--------+------+------+------+------+------+------+
| | I+ | I- | Q | I+ | I- | Q |
+--------+------+------+------+------+------+------+
| Winter | 0.29 | 0.21 | 1.49 | 0.33 | 0.27 | 1.22 |
| Summer | 0.39 | 0.28 | 1.34 | 0.44 | 0.39 | 1.13 |
+--------+------+------+------+------+------+------+
If the exceptional July values at Freiburg were omitted, the summer
values of I+ and Q would become 0.33 and 1.25 respectively.
18. _Diurnal Variation._--At Karasjok Simpson found the mean values of
I+ and I- throughout the whole year much the same between noon and 1
P.M. as between 8 and 9 A.M. Observations between 6 and 7 P.M. gave
means slightly lower than those from the earlier hours, but the
difference was only about 5% in I+ and 10% in I-. The evening values
of Q were on the whole the largest. At Freiburg, Gockel found I+ and
I- decidedly larger in the early afternoon than in either the morning
or the late evening hours. His greatest and least mean hourly values
and the hours of their occurrence are as follows:--
+-------------------------------+-------------------------------+
| Winter. | Summer. |
+-------+-------+-------+-------+-------+-------+-------+-------+
| I+ | I- | I+ | I- |
+-------+-------+-------+-------+-------+-------+-------+-------+
| Max. | Min. | Max. | Min. | Max. | Min. | Max. | Min. |
| 0.333 | 0.193 | 0.242 | 0.130 | 0.430 | 0.244 | 0.333 | 0.192 |
| 2 P.M.| 7 P.M.| 2 P.M.| 8 P.M.| 4 P.M.| 9 to | 4 P.M.| 9 to |
| | | | | |10 P.M.| |10 P.M.|
+-------+-------+-------+-------+-------+-------+-------+-------+
Gockel did not observe between 10 P.M. and 7 A.M.
19. Ionization seems to increase notably as temperature rises. Thus at
Karasjok Simpson found for mean values:--
Temp. less than -20 deg. -10 deg. to -5 deg. 10 deg. to 15 deg.
I+ = 0.18, I- = 0.36 I+ = 0.36, I- = 0.30 I+ = 0.45, I- = 0.43
Simpson found no clear influence of temperature on Q. Gockel observed
similar effects at Freiburg--though he seems doubtful whether the
relationship is direct--but the influence of temperature on I+ seemed
reduced when the ground was covered with snow. Gockel found a
diminution of ionization with rise of relative humidity. Thus for
relative humidities between 40 and 50 mean values were 0.306 for I+
and 0.219 for I-; whilst for relative humidities between 90 and 100
the corresponding means were respectively 0.222 and 0.134. At
Karasjok, Simpson found a slight decrease in I- as relative humidity
increased, but no certain change in I+. Specially large values of I+
and I- have been observed at high levels in balloon ascents. Thus on
the 1st of July 1901, at a height of 2400 metres, H. Gerdien (29)
obtained 0.86 for I+ and 1.09 for I-.
20. In 1901 Elster and Geitel found that a radioactive emanation is
present in the atmosphere. Their method of measuring the radioactivity
is as follows (48): A wire not exceeding 1 mm. in diameter, charged to
a negative potential of at least 2000 volts, is supported between
insulators in the open, usually at a height of about 2 metres. After
two hours' exposure, it is wrapped round a frame supported in a given
position relative to Elster and Geitel's dissipation apparatus, and
the loss of charge is noted. This loss is proportional to the length
of the wire. The radioactivity is denoted by A, and A=1 signifies that
the potential of the dissipation apparatus fell 1 volt in an hour per
metre of wire introduced. The loss of the dissipation body due to the
natural ionization of the air is first allowed for. Suppose, for
instance, that in the absence of the wire the potential falls from 264
to 255 volts in 15 minutes, whilst when the wire (10 metres long) is
introduced it falls from 264 to 201 volts in 10 minutes, then
10A = (254 - 201) X 6 - (264 - 255) X 4 = 342; or A = 34.2.
The values obtained for A seem largely dependent on the station. At
Wolfenbuttel, a year's observations by Elster and Geitel (56) made A
vary from 4 to 64, the mean being 20. In the island of Juist, off the
Friesland coast, from three weeks' observations they obtained only 5.2
as the mean. On the other hand, at Altjoch, an Alpine station, from
nine days' observations in July 1903 they obtained a mean of 137, the
maximum being 224, and the minimum 92. At Freiburg, from 150 days'
observations near noon in 1903-1904, Gockel (57) obtained a mean of
84, his extreme values being 10 and 420. At Karasjok, observing
several times throughout the day for a good many months, Simpson (10)
obtained a mean of 93 and a maximum of 432. The same observer from
four weeks' observations at Hammerfest got the considerably lower mean
value 58, with a maximum of 252. At this station much lower values
were found for A with sea breezes than with land breezes. Observing on
the pier at Swinemunde in August and September 1904, Ludeling (40)
obtained a mean value of 34.
Elster and Geitel (58), having found air drawn from the soil highly
radioactive, regard ground air as the source of the emanation in the
atmosphere, and in this way account for the low values they obtained
for A when observing on or near the sea. At Freiburg in winter Gockel
(55) found A notably reduced when snow was on the ground, I+ being
also reduced. When the ground was covered by snow the mean value of A
was only 42, as compared with 81 when there was no snow.
J.C. McLennan (59) observing near the foot of Niagara found A only
about one-sixth as large as at Toronto. Similarly at Altjoch, Elster
and Geitel (56) found A at the foot of a waterfall only about
one-third of its normal value at a distance from the fall.
21. _Annual and Diurnal Variations._--At Wolfenbuttel, Elster and
Geitel found A vary but little with the season. At Karasjok, on the
contrary, Simpson found A much larger at midwinter--notwithstanding
the presence of snow--than at midsummer. His mean value for November
and December was 129, while his mean for May and June was only 47. He
also found a marked diurnal variation, A being considerably greater
between 3 and 5 A.M. or 8.30 to 10.30 P.M. than between 10 A.M. and
noon, or between 3 and 5 P.M.
At all seasons of the year Simpson found A rise notably with increase
of relative humidity. Also, whilst the mere absolute height of the
barometer seemed of little, if any, importance, he obtained larger
values of A with a falling than with a rising barometer. This last
result of course is favourable to Elster and Geitel's views as to the
source of the emanation.
22. For a wire exposed under the conditions observed by Elster and
Geitel the emanation seems to be almost entirely derived from radium.
Some part, however, seems to be derived from thorium, and H.A.
Bumstead (60) finds that with longer exposure of the wire the relative
importance of the thorium emanation increases. With three hours'
exposure he found the thorium emanation only from 3 to 5% of the
whole, but with 12 hours' exposure the percentage of thorium emanation
rose to about 15. These figures refer to the state of the wire
immediately after the exposure; the rate of decay is much more rapid
for the radium than for the thorium emanation.
23. The different elements--potential gradient, dissipation,
ionization and radioactivity--are clearly not independent of one
another. The loss of a charge is naturally largely dependent on the
richness of the surrounding air in ions. This is clearly shown by the
following results obtained by Simpson (10) at Karasjok for the mean
values of a[+-] corresponding to certain groups of values of I[+-]. To
eliminate the disturbing influence of wind, different wind strengths
are treated separately.