19S Dike — Recent Observations in Atmospheric Electricity. 



is the potential gradient — the existence of an electric field about 

 the earth, resulting in a difference of potential between the 

 earth and an} T point in the air above it. The existence of such 

 a potential is readily shown by the use of a suitable electro- 

 scope or electrometer connected to a collector, such as a flame, 

 a water dropper or a radio-active body. Potential gradients 

 exceeding 100 volts/meter are ordinary, and during times of 

 electrical disturbance, even with a clear sky, this may increase 

 to 1000 volts or more. The mean value at Kew, England, has 

 for eight years exceeded 159 volts per meter, while on January 

 1, 1908, during a fog it exceeded 730 continuously for 8-J hours. 

 Its fluctuations are abrupt and of great range, sometimes passing 

 suddenly from large positive to negative values. At Kew, at 

 Potsdam, and at various other observatories, continuous records 

 have been kept of this most erratic phenomenon by means of 

 self-recording instruments for years, in an attempt to establish 

 some relationship between it and other meteorological elements, 

 but so far without very definite results, except for a possible 

 connection with the barometric pressure ; and this relationship, 

 according to Chree, seems to point to change of potential 

 gradient as a cause, and change of pressure as a result. It 

 would seem to be almost impossible to correlate such a phenome- 

 non with local weather conditions except in so far as electri- 

 cal storms are concerned, when it is considered that as a charged 

 body we have the earth, whose electrical condition is depend- 

 ent on conditions prevailing over its whole surface rather than 

 on local phenomena. 



By recent observers the study of this element of the problem 

 has been made subsidiary to other investigations in which it is 

 involved as a factor, principally in the computation of the 

 earth-air currents. 



An interesting series of investigations with a practical end 

 in view has been made by Creighton of the General Electric 

 Company bearing upon the subject of electrical storms with 

 especial reference to lightning discharges and the operation of 

 lightning arresters on transmission lines. Laboratory experi- 

 ments were supplemented by observations at two power plants 

 in Colorado where electric storms are of daily occurrence in 

 summer. By means of a rotating film or a moving sensitive 

 tape the duration of a discharge on a transmission line was 

 measured by means of projecting the light from a spark gap 

 connected with the line upon the film. A range of duration 

 of from 0*04 sec. to 0*0001 sec. was found in 36 discharges, the 

 greater number being of very short duration and only two 

 exceeding 0*01 sec. As many as seven discharges in a second 

 were observed. The discharge is oscillatory, showing 3000 

 cycles per second, corresponding to the line frequency, and 



