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



removed and the cover put on in its place. The potential of 

 the system can then be obtained from the gold-leaf reading, while 

 the charge is known, since it is that which was held on the upper 

 surface of the test-plate when at the same potential as the guard 

 ring (i.e. zero), and with the condenser plate at a known potential 

 above it. The capacity of the test-plate and gold-leaf system 

 with the ordinary cover on and with the compensator drawn out 

 was about 9'5 cms.; with the compensator in it was about 10 per 

 cent, greater. A knowledge of the capacity of the system was not 

 required in the measurements of potential gradient or earth-air 

 currents. It is given for the purpose of indicating the order of 

 magnitude of the smallest charge measurable with the instrument. 

 The compensator could be adjusted to give coincidence between 

 the edge of the gold-leaf and a scale division to within 1/20 of a 

 scale division, i.e. to within 1/120 of a volt, corresponding to a 

 quantity less than 1/120 x 1/300 x 9'5 = about 1/3000 E.u. 



Most of the measurements of the charge upon and current 

 through the test-plate when kept at zero potential have been 

 carried out on the top of Hamildon Hill, near Peebles in Scotland. 

 The hill is 1210 feet (370 metres) high, and has a flattened top. 

 The electrometer stood upon a metal tripod adjusted to such a 

 height that the test-plate was either 60 cms., 90 cms., or 130 cms. 

 above the ground. I have not attempted to determine the factor 

 by which the charge on the test-plate must be multiplied to give 

 the undisturbed potential gradient or the charge per square cm. of 

 the ground. The order of operations generally adopted was as 

 follows. When the apparatus had been set up the compensator 

 constant (the factor by which the ordinates of the calibration 

 curve had to be multiplied to obtain the corresponding charge in 

 electrostatic units) was first determined by finding the displace- 

 ment of the compensator required to cause a movement of the 

 gold-leaf through a definite number of scale divisions as described 

 above. The test-plate system was then earthed by means of the 

 earthing key and the electrometer zero observed ; it was adjusted 

 by a suitable displacement of the microscope so that the straight 

 edge of the gold-leaf coincided exactly with one of the scale 

 divisions of the eye-piece scale. The earth connection was then 

 broken, and at a definite time the cover was removed and the com- 

 pensator at once moved to bring the potential back to zero. The 

 compensator reading was then read, this giving the charge on the 

 plate. The compensator was moved as might be required to keep 

 the potential at zero, and when a definite number of minutes, 

 generally from two to five, had elapsed since its removal, the 

 cover was replaced and the compensator drawn out to bring the 

 potential back to zero, its reading then giving the charge gained 

 by the plate in the known time. The system was then earthed to 



