abstracts: atmospheric electricity 15 



nitude over the sea as over the land; the radioactivity of the air over 

 ocean-areas far removed from land is small, compared to that found 

 over land; and the ionization over the ocean is at least as large as that 



found over land. 



C. W. H. 



ATMOSPHERIC ELECTRICITY.— Investigation of certain causes 

 responsible for uncertainty in the measurement of atmospheric con- 

 ductivity by the Gerdien conductivity apparatus. C. W. Hewlett. 

 Terr. Mag., 19: 219-233. 1914. 

 The object of the writer was to test experimentally the behavior of 

 the apparatus under various conditions. It was shown theoretically 

 by Dr. Swann that the theory of the apparatus does not necessitate 

 a constancy of the velocity over a cross section of the air current, 

 and that, if the potential is below a certain minimum value depending 

 on the total air flow, the correct value of the conductivity is obtained. 

 By covering up half of the space opposite the fan, a considerable 

 irregularity was produced in the air flow. For low charging voltages, 

 however, the conductivity was practically the same as without the 

 obstruction. As the voltage is increased beyond a certain point, the 

 measured conductivity, without the obstruction, becomes less than 

 that with it, but this is due to the fact that the critical voltage is less 

 with the obstruction than without it, owing to the smaller air flow in 

 the former case. 



As the measured conductivity was found to depend upon the pres- 

 ence or absence of the funnel, it was thought advisable to investigate 

 the exact effect of the latter. Certain theoretical considerations point 

 to the conclusion that, in the absence of the funnel, the maximum 

 allowable voltage is reduced, owing to the fact that the stream lines 

 of the air which enter the funnel near the edges suffer sharp bends in 

 that vicinity. Experiments were consequently made to determine 

 whether the effect of the funnel was less important for low charging 

 voltages. It was found that for charging voltages of 10, 30, 50, 70, 

 100, the relation of the conductivity without the funnel to that with 

 the funnel was 0.98, 0.96, 0.98. 0.94, 0.89; hence the theoretical con- 

 clusion that the ratio is practically unity for low voltages, and less 

 than unity for higher voltages, is borne out. Experiments have also 

 been made to determine the effect of the charge induced on the earthed 

 portions of the apparatus owing to the existence of the potential gra- 

 dient. It is found that under certain conditions this effect may be 



