( 395 ) 
ON THE VAEIATION IN THE VALUE OF THE ATMOSPHEEIC 
ELECTRICAL POTENTIAL WITH THE ALTITUDE. 
By W. a. Douglas Rudge, M.A. 
(Read March 20, 1911.) 
Observations taken during the past two years have convinced the 
v^riter that some sort of relation should exist between the value of the 
potential gradient due to the charge existing in the air and the altitude of 
the place of observation, and some account of this matter has already been 
published by him. With the object of obtaining further information, a 
journey was made in September last from Bloemfontein to Lourengo 
Marques via Johannesburg, and then from Johannesburg to Durban. In 
this way places of observation could be secured where the altitude varied 
from sea-level up to nearly 7,000 feet, and records were taken of the 
potential gradient at as many places as possible. 
The apparatus used consisted of a " Wilson " pattern gold-leaf electro- 
meter, furnished with a collecting-plate of brass coated with a radium 
preparation. This latter had been in use for some time and gave 
consistent results, but ionium would probably be a better material than 
radium to work with. The results may, however, be regarded as com- 
parative rather than absolute, and tests of the efficiency of the plate at 
the beginning and end of the observations showed that it had not 
changed in its power of taking the potential of that layer of the 
atmosphere in which it happened to be. The electroscope was calibrated 
by a number of small storage cells used in conjunction with a sliding 
condenser, allowing potentials up to 500 volts to be measured. The 
electroscope was usually arranged so that the collecting plate was 140 cm. 
above the ground, but in some cases where the potential was high a less 
height was used. The movements of the gold leaf were observed by a 
reading microscope with a scale in the eyepiece. The gold leaf rapidly 
took up a fixed position (in a uniform field) and the double case of the 
instrument effectually eliminated any disturbance by the wind. 
