ATMOSPHERIC ELECTRICITY. 233 



attached a small plate covered with a radium preparation. The 

 instrument was mounted upon a telescopic stand, so that the 

 height of the plate above the ground could be varied. From a 

 plate coated with a radioactive preparation a stream of ions will 

 shoot off, land, if the plate is in an electric tield, it will rapidly 

 take the same potential as that portion of the field in which it 

 is resting. The same thing happens to a wire to which a piece 

 of smouldering tinder is attached, and also to an insulated vessel 

 furnished with a long tube from which water may be dropped, as 

 used originally by Lord Kelvin. The radium preparation, whilst 

 open to certain objections, is undoubtedly the most convenient, 

 especially when taking ()l)servations wdiilst moving from place 

 to place. 



The method of observation consisted in setting up the electro- 

 scope and adjusting the height of the plate above the ground 

 imtil a readable deflection of the gold leaf was seen. The leaf 

 was observed from a short distance by an opera-glass, so that 

 the presence of the observer should not interfete. In some cases 

 the gold leaf diverged so far that it touched the earthing plate, 

 so that in these, the potential could not be found directly, but 

 had to ibe inferred from the rate at which the charge rose to the 

 maximum. The sensitiveness of the instrument would be varied 

 from I division for lO volts to^ i division for 26 volts, the whole 

 range running up to about 600 volts. The gold leaf rapidly took 

 up its position, and, if no wind was blowing, remained steady, 

 so that observations Avere easily taken. 



In comparing observations at dift"erent places, it is essential 

 to make use of those taken al corresponding periods of the day. 

 This w^as not always possible, but by "smoothing" curves and 

 taking averages, it was possible to get a fairly good result for 

 10 a.m., the time usually tiaken for comparison purposes. 



In May. 1910, an extended series of observations was taken 

 at the time of passage of Halley's Comet near to the earth. 

 During that time the potential gradient varied from about 200 to 

 40 volts per metre. The value frooi day to day was fairly 

 constant with maxima occurring at about 8 a.m. and 7 p.m. 



The table and curve show the variations during a day. 



Table Showing One Day's Observation at Bloemi^-ontein. 

 4570 ft. ABOVE Sea-level. 



Potential Potential Potential 



Time. Gradient. Time. Gradient. Time. Gradient. 



oa.m 38 9.20 163 10.45 130 



6. 32 9.25 163 II 



6.30 36 9-35 185 II 



7.30 40 Q.40 208 1 1 



8. 66 9.4s 217 II 



8.20 8g 9.47 .. .... .. 210 II 



8.30 103 10. 210 II 



8.40 loq 10.10 255 II 



8.56 136 10.20 235 II 



9.0 145 TO.30 163 II 



9.10 148, 10.35 140 II 



114 



I 92 



86 



8 70 



10 66 



[5 60 



20 56 



22 52 



25 52 



35 52 



