Charge (unit =2 ‘2 1 9 x 10 -2 E.S.U. ). 
1909-10.] Observations of the Earth-air Electric Current, etc. 461 
plate is surrounded by an earthed guard-ring, 16 - 8 cms. external diameter, 
the annular gap between being 023 cms. wide. A brass cover rests on the 
guard-ring and shields the test-plate ; a horizontal metal rod connected to 
the vertical rod of the gold-leaf system is surrounded by a brass tube kept 
at a constant potential by another quartz Leyden jar ; this system forms a 
cylindrical condenser — the compensator — the capacity of which can be 
varied by moving the brass tube parallel to its length. 
The calibration curve, which gives the charge to be measured from the 
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 
Compensator Reading. 
Fig. 2. 
compensator readings, was found to approximate very closely to a straight 
line, as in the case of Wilson’s later apparatus. Fig. 2 shows a calibration 
curve, the abscissse being compensator readings and the ordinates charges. 
The form of this curve is independent of the potential of the compensator 
tube, this potential being constant during the determination of the curve ; 
and to allow for a change of potential, the ordinates have merely to be 
multiplied by a factor, viz. the ratio of the new potential to the old 
potential. This ratio can be obtained from the deflection of the gold leaf 
produced by a given motion of the compensator in the two cases. (If the 
compensator be moved inwards, say from 0 to 30, the gold-leaf system 
