AT'MOSPHER'IC E'LECTRIC'ITY. Ob servation shows that the lower atmosphere, with its clouds and rain, or snow and dust, is gener ally in an electrified state. In November, 1749, Benjamin Franklin argued that lightning is but an immense electric flash, similar to the sparks obtained from electrical machines. On May 10, 1752, Billion and d'Alibard experimentally con firmed this hypothesis by the use of a lightning rod near Paris: and in June, Franklin himself, by means of a kite raised during a thunderstorm at Philadelphia, was able to draw electric sparks from the clouds and the atmosphere. Since that day an immense number of observa tions have been made upon the various electric phenomena of the air. Regular observations of atmospheric electricity were originally made, either by means of kites carrying insulated wire from the ground up toward the clouds, or more conveniently by means of metal rods insulated from the ground by resin, or glass, or silk; the upper end of the rod was sometimes pointed, but more properly ended in a sphere ; the lower end was connected with a pair of gold-leaf strips, as in the earliest form of electroscope. This appa ratus was improved by attaching a slow-burning match or flame at the upper end of the rod. The best of the early forms of apparatus was that of Hellman, in which a brass hall is electrified by induction and afterwards its exact condition is determined by applying an electrometer. The modern method devised by Sir William Thom son (Lord Kelvin), about 1860, consists of a reservoir of water, supported on insulating glass pedestals; a small insulated tube leads from the reservoir to some point in the open air, at a dis tance from the building, and the water flowing through this tube forms a stream that is broken into drops within a few inches of the end of the tube. This constitutes the water-dropping col lector. In freezing weather the water must be replaced by a vessel of oil, with wick and flame. The most modern apparatus acting by induction is that devised by Mr. Morrill, of the United States Weather Bureau. If the electric potential of the tube and the water at the point where the stream breaks into drops differs from that of the adjacent air, the drops will carry away the positive or negative excess, and soon bring the collector to the same potential as that of the air. The electric
condition is observed by measuring, not sparks or currents, but the difference of potential that tends to cause such currents. For brevity, the word 'potential' is used to signify difference of potential. The difference of potential between the air and the earth is measured by connecting the ground and the collector to the opposite poles of sonic form of electrometer, and for this pur pose the quadrant electrometer of Kelvin has been most widely used, since it was first intro duced at the Kew Observatory in 1861. A modification of all this apparatus has been used in France since 1875, known as Mascart's sys tem, and this apparatus has also been used by the United States Weather Bureau. In 1884 that bureau inaugurated a systematic inves tigation of this subject by starting observations and studies under the guidance of Professors Rowland, at Johns Hopkins, and Trowbridge, at Harvard University. This work was subsequent ly placed under the control of Prof. T. C. Men denhall, whose report was published in the Mem oirs of the National Academy of Sciences, Vol. V. (1889). The specific object of this investigation was to determine whether there was any apparent connection between the electrical condition of the atmosphere and the development and prograss of storms, so that such observations could be util ized in the improvement of storm and weather predictions, or in drawing lines of equal electric potential on the Daily Weather Map. It was, however, demonstrated that the potential de pends so much upon local conditions that observa tions made in the same neighborhood gave entire ly different results, and that it was necessary to learn how to interpret the local record and its oscillations before combining it with records from another station. As it did not seem likely that results of any value in weather forecasting could be secured at present, the further prosecution of the subject was discontinued. The principal work that has been done since 1887 has been that of Elster and Geitel in Germany, Mascart and Cha veau in France, and J. J. Thomson and C. T. R. Wilson in England.