408 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
! 
REPORT UPON ATMOSPHERIC ELECTRICITY. 
BY W. F. MAGIE, PH. D. 
Professor of Phydcs in the College of New fersey, Princeton. 
The observations on atmospheric electricity presented in the following report were 
made by the author during the summer of 1880 on board the U. S. Fish Commission 
schooner Grampus, while he was engaged in assisting Professor Libbey in the investi- 
gation of ocean temperatures. The duties of the regular work were so exacting that 
no systematic record of the atmospheric electricity was kept ; but the observations 
were made to decide whether such a record would be valuable. On this point the f 
observations, few as they are, leave no question. It is stated by Exner, in his | 
elaborate paper, Ueber die Ursache und die Gesetze der Atmospharischeu Elektricitat, f 
Wien. Sitzber., Bd. xoiii, Abth. ii, 1886, that useful observations on the normal elec- | 
trical state of the atmosphere during the summer months are frequently rendered ‘ 
impossible by reason of the disturbances caused by dust particles in the air. The ; 
clouds of flying dust often change the normal potential of the air, which increases a 
from the earth’s surface upwards, to a negative potential, and even when this reversal | 
of potential does not occur, must materially affect the amount of the positive increase. 
No such disturbances could be traced in the potentials observed at sea. They were 
invariably positive. Observations at sea are free also from any constant error, such 
as affects those made on land, due to irregularities in the configuration of the earth’s 
surface and to the presence of buildings and trees. The effect due to the vessel can 
be determined by a special observation and allowed for in all subsequent work. It 
is only necessary to make simultaneous observations on the vessel and in a boat at 
some distance from it, to obtain data for reducing all potentials observed on the vessel 
' 
to the true potentials at the sea level and referred to the earth’s sphere. Hence, if 
we adopt Exner’s view that the earth is negatively charged and that its charge is a 
cosmical constant which can be determined by observation of the rate of change of 
potential at the earth’s surface, the data from which this charge can be most accu- 
rately calculated must be those obtained at sea. Observations at sea are also very 
well adapted to test the relation between the potential and the humidity of the 
atmosphere and offer exceptional facilities for the study of the effect of clouds. i 
The observations were made with a Thomson’s portable electrometer, which was: 
found to be much more sensitive and accurate than the portable gold-leaf electrometer . ^ 
used by Exner. The motion of the vessel did not interfere in any way with the 
accuracy of the readings. It was easy to repeat a setting of the attracting plate to:, 
within one division of the micrometer head. This insured an accuracy of measure - 4 
ment far greater than is necessary in this class of observations, in which so many ! 
unexplained fluctuations are constantly occurring. The instrument was charged ‘ 
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