366 Mr Wilson, On the Measurement of the Earth-Air 



terminal of the electrometer is a horizontal circular brass " test- 

 plate," 5 cms. in diameter. This is surrounded as shown by a 

 guard ring 15 cms. in external diameter, the upper surfaces of 

 test-plate and guard ring being in the same horizontal plane. The 

 annular gap between the test-plate and the guard ring was half a 

 cm. wide. The guard ring was supported on the top of a brass 

 cylinder 25 cms. high and 8"5 cms. in diameter, the base of this 

 being carried by a tube, about 5 cms. long and 1*5 cms. wide, which 

 fitted on to the vertical tube attached to the top of the electro- 

 meter case and surrounding its terminal. On the guard ring 

 could be placed the " cover " of brass, 4 cms. high and 8*5 cms. in 

 diameter. 



In making a measurement the electrometer is placed on a 

 low tripod stand in an open space, the surface of the test-plate 

 being in most cases from 60 to 100 cms. above the ground. The 

 cover is placed on the top, and the system of test-plate, vertical 

 supporting rod and gold-leaf earthed by means of the earthing rod. 

 When it is desired to make a measurement the earth connection 

 is broken and the cover removed. The observer with his eye at 

 the observing microscope will see the gold-leaf deflected from its 

 zero position when the cover is removed, and he at once brings it 

 back to zero by sliding in the compensator ; this adjustment can 

 be made within a very few seconds. The reading of the compen- 

 sator, if it has previously been standardised, will give at once the 

 charge on the test-plate when exposed to the earth's field and kept 

 at zero potential. By adjusting the compensator as may be re- 

 quired the observer now maintains the gold-leaf as nearly as 

 possible at the reading corresponding to zero potential for a few 

 minutes, and the cover is replaced after a definite number of 

 minutes have elapsed since its removal. The compensator is now 

 drawn out till the gold-leaf again indicates zero potential. If no 

 current had entered the test-plate from the atmosphere the com- 

 pensator would have had to be drawn right out to its initial 

 position ; the actual compensator reading when the zero potential 

 is indicated by the gold-leaf enables the charge gained by the 

 plate in the given time to be deduced. The test-plate is again 

 earthed, the compensator drawn out to its standard position, the 

 earth connection broken, the cover removed and the charge on the 

 exposed test-plate when brought to zero potential again measured ; 

 under ordinary conditions the mean charge on the test-plate 

 during the exposure may be taken as the mean of the results 

 of the initial and final observations, which will of course be 

 identical if the potential gradient has not meanwhile altered. 

 Any rapid or irregular changes in the potential gradient during 

 the exposure will be detected through the movements which have 

 to be given to the compensator to keep the potential at zero. 



