Dike — Recent Observations in Atmospheric Electricity. 203 



change of charge, that is the loss or dispersion of electricity 

 from the exposed test plate, can be measured by reading the 

 scale before and after exposure, the gold leaf being kept con- 

 stantly at its zero position by means of a gradual motion of 

 the compensator. 



The test plate being covered, the gold-leaf system is earthed 

 and the position of the compensator is read. The earth con- 

 nection is then broken and the cover removed. The observer 

 then keeps the gold leaf at its zero position, that is, keeps the 

 test plate at zero potential by means of the compensator till a 

 measured interval of time has elapsed, when the position of 

 the compensator is again read. From the difference of the 

 two readings the loss of charge can be computed and thus the 

 current per square cm. of the test plate determined. Alternate 

 measurements were made with the bare test plate and with a 

 piece of turf covering the plate to represent a section taken 

 from the surface of the earth. The dissipation factor is 

 expressed as the percentage of the charge on an earth-connected, 

 exposed body which is neutralized per minute. The dissi- 

 pation factor for the plate alone was found to be practically 

 the same as for the turf, though the charge on the turf was 

 about three times as great and the distribution of charge very 

 different. The agreement is sufficiently good to warrant assum- 

 ing a definite dissipation factor depending on the condition of 

 the atmosphere. In order to get a closer approximation to 

 actual conditions at the surface of the ground, instead of at the 

 elevated test plate, a large wooden test plate and guard ring 

 with their surface very little above the surface of the ground 

 were constructed and connected with the test plate of the appar- 

 atus. Alternate determinations were made with the small and 

 the large test plates, precautions being taken that the field 

 should not be disturbed by the observer. From those observa- 

 tions it was found that the mean density of electrification upon 

 the exposed test plate when at zero potential was nearly 4*2 

 times that upon the surface of the ground. Using this reduc- 

 tion factor, observations extending over a year give a mean 

 value for the current per square cm. of ground 2'2XlO~ 16 am- 

 peres, agreeing with Gerdien's value 2'7X 10" 16 amperes deduced 

 from measurements of conductivity and potential gradient. 

 The dissipation factor was least for cloudless, calm days, great- 

 est for days with cumulus and clear atmosphere, and inter- 

 mediate for overcast days. The agreement between the two 

 widely different methods is doubtless partly accidental, but is 

 none the less gratifying as indicating that definite results are 

 being obtained which can be duplicated by different observers 

 using different methods. 



