Page:EB1911 - Volume 02.djvu/910

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862
ATMOSPHERIC ELECTRICITY
1904 varied from 141 in 1900 to 179 in 1899, the mean from the seven years combined being 159. The large difference between the means obtained at Potsdam and Kremsmünster, as compared to the comparative similarity between the results for Kew and Karasjok, suggests that the mean value of the potential gradient may be much more dependent on local conditions than on difference of latitude.
At any single station potential gradient has a wide range of values. The largest positive and negative values recorded are met with during disturbed weather. During thunderstorms the record from an electrograph shows large sudden excursions, the trace usually going off the sheet with every flash of lightning when the thunder is near. Exactly what the potential changes amount to under such circumstances it is impossible to say; what the trace shows depends largely on the type of electrometer. Large rapid changes are also met with in the absence of thunder during heavy rain or snow fall. In England the largest values of a sufficiently steady character to be shown correctly by an ordinary electrograph occur during winter fogs. At such times gradients of +400 or +500 volts per metre are by no means unusual at Kew, and voltages of 700 or 800 are occasionally met with.
5. Annual Variation.—Table I. gives the annual variation of the potential gradient at a number of stations arranged according to latitude, the mean value for the whole year being taken in each case as 100. Karasjok as already mentioned is in the extreme north of Norway (69° 17′ N.); Sodankylä was the Finnish station of the international polar year 1882-1883. At Batavia, which is near the equator (6° 11′ S.) the annual variation seems somewhat irregular. Further, the results obtained with the water-dropper at two heights—viz. 2 and 7.8 metres—differ notably. At all the other stations the difference between summer and winter months is conspicuous. From the European data one would be disposed to conclude that the variation throughout the year diminishes as one approaches the equator. It is decidedly less at Perpignan and Lisbon than at Potsdam, Kew and Greenwich, but nowhere is the seasonal difference more conspicuous than at Tokyo, which is south of Lisbon.


Table II.—Diurnal Variation Potential Gradient.

Station. Karasjok. Sodankylä. Kew (19, 12). Greenwich. Florence. Perpignan. Lisbon. Tokyo. Batavia. Cape

Horn (20).

Period. 1903-4. 1882-83. 1862-
1864.
1898-
1904.
1893-96. 1883-85. 1886-88. 1884-86. 1897-98
1900-1.
1887-
1890.
1890-
1895.
1882-83.
Days. All. All. Quiet. All. All. Fine. All. All. Dry. Dry. Pos.
h
l
5.5 3.0
2.5
3.5
1.0
3.35
1.3
3.0
1.8
8.4
1.5
3.0
0.5
1.7
2.0
2 7.8 3.5
2.0
Hour.
1
2
3
4
5
6
7
8
9
10
11
Noon.
1
2
3
4
5
6
7
8
9
10
11
12

  83
  73
  66
  63
  60
  68
  81
  87
  94
101
  99
103
106
108
108
109
110
119
129
136
139
133
121
102

  91
  85
  82
  84
  89
  91
  97
100
  98
102
  98
102
105
107
108
108
108
110
102
111
111
104
108
  93

  87
  79
  74
  72
  71
  77
  92
106
107
100
  90
  92
  90
  91
  92
  98
108
121
134
139
138
128
113
  99

  93
  88
  84
  83
  85
  93
103
112
115
112
101
  94
  89
  87
  88
  93
  99
108
115
118
119
115
108
  99

  97
  89
  87
  86
  86
  92
100
102
100
101
  96
  97
  96
  94
  95
  97
102
108
111
115
117
117
111
104

  92
  83
  77
  75
  74
  82
100
112
113
107
100
  95
  92
  90
  89
  89
  94
113
121
129
132
127
114
100

  78
  72
  71
  72
  77
  92
107
114
111
100
  96
  99
  99
  97
  99
105
113
126
131
129
120
109
  97
  86

  84
  80
  78
  81
  83
  92
101
105
104
104
102
108
111
114
109
108
108
111
116
114
109
102
  92
  85

101
  98
  97
  99
121
154
167
149
117
  87
  70
  61
  54
  49
  53
  61
  76
  95
107
114
119
120
119
112

147
141
135
128
127
137
158
104
  67
  42
  35
  30
  30
  30
  33
  41
  67
  91
120
137
146
148
151
147

125
114
109
102
101
117
147
119
  82
  55
  46
  43
  42
  43
  46
  53
  73
108
145
155
155
147
143
130

  82
  73
  85
  81
  85
  95
106
118
119
123
123
115
112
  94
  89
  88
  84
110
107
123
112
  99
  85
  98


Table III.—Diurnal Variation Potential Gradient.

Station. Karasjok. Sodankylä. Kew. Greenwich. Bureau
Central (21).
Eiffel
Tower (21).
Perpignan (21). Batavia.
(2 m.)
Period. 1903-4. 1882-83. 1898-1904. 1894 and ’96. 1894-99. 1896-98. 1885-95. 1887-90.
Winter.
Summer.
Winter.
Summer.
Winter.
Equinox.
Summer.
Winter.
Summer.
Winter.
Summer.
Summer.
Winter.
Summer.
Winter.
Summer.
Hour.
1
2
3
4
5
6
7
8
9
10
11
Noon.
1
2
3
4
5
6
7
8
9
10
11
12

  76
  66
  57
  55
  50
  61
  78
  82
  90
104
102
119
116
118
119
115
120
131
136
134
137
125
114
  96

104
  96
  89
  83
  79
  83
  89
  93
  93
  93
  92
  90
  94
  97
100
  99
106
104
110
113
125
135
126
111

  90
  79
  78
  74
  74
  80
  86
  95
  91
106
  98
  98
116
113
121
111
105
115
118
117
115
112
113
  95

  99
  84
  90
  99
111
114
117
122
109
101
  97
100
  97
  97
  93
  96
106
  92
102
106
  90
  90
103
  85

  91
  86
  82
  81
  82
  86
  95
104
111
114
107
102
  99
  97
  99
103
108
111
114
112
111
108
103
  96

  93
  88
  85
  84
  87
  97
109
118
119
110
  95
  86
  81
  80
  82
  88
  96
109
120
124
123
118
109
  99

  96
  90
  85
  84
  90
101
113
120
119
110
  97
  87
  80
  76
  76
  80
  87
  98
111
123
129
125
116
105

  87
  84
  76
  77
  78
  82
  94
  97
  98
102
103
107
107
109
111
116
112
114
117
113
111
110
102
  93

110
101
  98
  96
  94
101
107
111
102
  98
  86
  94
  85
  82
  78
  81
  93
  98
  99
108
118
124
120
116

  79
  71
  70
  69
  75
  83
  98
111
113
111
108
106
112
112
111
113
120
124
124
116
104
  97
  90
  83

102
  92
  88
  84
  94
106
118
120
106
  94
  84
  77
  79
  81
  78
  80
  85
  97
123
134
130
122
115
108

  90
  83
  79
  76
  78
  87
  97
103
110
109
107
104
107
110
107
105
106
109
113
110
109
105
101
  94

  72
  67
  66
  67
  72
  84
104
122
126
114
  98
  99
  96
  94
  95
102
115
128
133
131
124
111
  96
  83

  88
  83
  81
  83
  92
107
114
108
100
  93
  90
  95
  93
  90
  88
  92
  98
110
122
127
125
117
108
  95

145
139
137
131
132
138
166
118
  74
  43
  35
  31
  29
  28
  24
  30
  60
  88
119
138
145
148
149
148

149
142
135
127
123
136
153
  92
  64
  40
  36
  30
  33
  32
  41
  49
  74
  94
122
135
147
148
152
146