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 |
