200 Dike — Recent Observations in Atmospheric Electricity, 



a distance from the charged body, whose loss of charge is to be 

 measured, that at the potential to be used the currents shall be 

 unsaturated, or by setting up a current of air about the charged 

 body of such velocity that the same result is reached. 



The latter, first applied in its fullest extent by Gerdien, has 

 proved to be of great use for field work, while the former 

 seems more suitable for observatory use, where continuous 

 records are desired. The Gerdien conductivity apparatus, 

 briefly described, consists of a cylindrical condenser, the outer 

 cylinder of which is 16 cm in diameter and 35 cm long, while the 

 inner cylinder is l*4 cm in diameter and 24 cra long, and is con- 

 nected with an electroscope by means of which the potential 

 of a charge put upon the cylinder can be observed. A current 

 of air, of sufficient velocity to prevent the establishment of 

 saturation currents, is drawn through the condenser with the 

 aid of a fan, thus using the second of the two methods of avoid- 

 ing saturation. This instrument has been used by various 

 observers on land, in balloons and at sea, with very consistent 

 results, and it has proved very satisfactory for field work. 

 Gerdien has made use of it at Gottingen in conjunction with 

 a self-recording instrument giving the course of the potential 

 gradient for determining the value of the earth-air current. 



The potential gradient and the conductivity being known, 

 the current per square centimeter of the earth's surface i& 

 readily computed. 



bv /x x . 



A series of observations carried out during about two and 

 a half weeks in April, 1906, gave as a mean from 49 measure- 

 ments 8 X 10~ 7 electrostatic units per square cm. for the 

 value of the vertical current, or 2'7xlO~ 1G amperes. The con- 

 ductivity was found to be X / =1'155 X 10~ 4 and X w =l*120 X 

 10" 4 electrostatic units. 



I have made use of the same type of instrument at sea, on 

 board the Galilee during the cruise recently completed, begin- 

 ning at Sitka and ending at San Francisco, obtaining observa- 

 tions, mostly in fair weather conditions, in latitudes ranging 

 from 55° 41/ North to 45° 07' South, crossing the tropics twice, 

 thus securing a wide range of climatic conditions. Unfortu- 

 nately on board ship I was unable to measure the normal 

 potential gradient in an undisturbed field, and the disturbances 

 were so great and so variable that it seemed impossible to avoid 

 or to correct for them. Hence my observations give only the 

 conductivity and the potential gradient can only be surmised. 

 A few observations made in a skiff at sea in very calm weather 



