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LXXVI. Earth-Aii' Electric Currents. By George C. 

 Simpson, D.Sc, Government of India Meteorological 

 Department *. 



CONSIDERING the present state of our knowledge of 

 atmospheric electricity, perhaps the most important 

 factor requiring study is the current of electricity which 

 is constantly passing between the earth and the air. This 

 current has already been investigated to some extent, and 

 two methods have been used for the purpose. Gerdienf 

 determined the current by measuring the potential gradient 

 and the conductivity of the air; and Wilson J has investigated 

 it by observing the actual loss of electricity from a small 

 plate exposed to the atmospheric field and kept at zero 

 potential. The measurements made by these two methods 

 have only been few; and in view of the large variations 

 which the current undergoes in the course of a few minutes, 

 the need has been strongly felt for some method by means of 

 which the current could be automatically recorded. 



Neither Wilson's nor Gerdien's methods lend themselves 

 for automatic registration, but an instrument has been 

 developed in the Meteorological Office, Simla, which has 

 proved entirely satisfactory in use. If we confine ourselves 

 to the consideration of normal conditions during fine weather, 

 the problem may be stated thus : — ■ 



Consider a small area of the earth's surface. On account 

 of the electrical field in the atmosphere there is an induced 

 surface charge of negative electricity on the area. This 

 charge suffers change on account of two causes: (1) Every 

 change in the potential gradient is accompanied by a change 

 in the induced charge — the greater the potential gradient 

 the greater the charge, and vice versa. (2) The electrical 

 field in the atmosphere is constantly driving free positive 

 ions from the air on to the surface ; or, as it is generally 

 expressed, the conductivity of the air results in a current of 

 negative electricity from the earth into the air. 



As the area under consideration forms part of the surface 

 of the earth, its potential remains zero ; hence each change 

 of charge from either of these causes gives rise to a flow of 

 electricity between the area and the earth. If, on the other 

 hand, the area were insulated its potential would be constantly 

 undergoing change unless steps were taken to keep it zero, 

 by either adding or withdrawing a charge similar in amount 

 to that which would have flowed to or from it if it had been 



* Communicated by the Physical Society : read March 11, 1910. 



t Gerdien, Gotting. Nachr. i907, p. 77. 



\ C. T. R. Wilson, Proc. Roy. Soc. A. lxxx. p. 537 (1908). 



