Current and on the Origin of Atmospheric Electricity. 373 



time by suitably charging the Leyden jar attached to the compen- 

 sator, or the new reduction factor to be applied to the calibration 

 curve could be determined by an observation like that just 

 described. Thus for example after the above calibration was 

 performed the compensator potential was diminished so that to 

 cause, under the standard conditions, a deflection of the gold-leaf 

 amounting to 5 scale divisions the compensator, originally fully in, 

 had to be drawn out through 9'8 mm. The reduction constant 



would thus be increased in the ratio —=-: -^pr = 9 "3. A direct 



5 «-$(J 



determination of the compensator readings corresponding to known 



charges gave the results shown in the following table, the charges 



being given in the same units as in the preceding table. 



Charge 



Compensator 

 reading 



6. 



1 

 6 

 1 

 6 

 1 

 2 



4-3 

 20-9 



4-3 

 20-9 



4-4 

 20-9 

 17-6 



To give a reading 43 now given by a charge of 6 arbitrary 

 units a charge of 50 units would before have been required, the 

 ratio being 56/6 = 9 # 3, in agreement with that just obtained 

 indirectly. For the compensator readings 17*6 and 20"9 the ratios 

 are 9'4 and 9 '5 respectively. 



The sensitiveness of the electrometer was 6*0 eye-piece scale 

 divisions per volt. The microscope was turned so that the rulings 

 on the scale were parallel to the edge of the gold-leaf; under these 

 conditions when a straight portion of the edge of the gold-leaf 

 was used the coincidence of the edge of the gold-leaf with a line 

 on the scale could be observed with great exactness; a departure 

 from coincidence amounting to a small fraction of a scale division, 

 certainly less than a twentieth, could readily be observed. 



The capacity of the system which included the test-plate and 

 gold-leaf was easily determined. For this purpose the condenser 

 plate was placed on the guard ring as in standardising the com- 

 pensator, and was raised to a known potential by means of a 

 number of cells, the gold-leaf system being meanwhile earthed. 

 The earth connection was then broken, the condenser plate 



VOL. XIII. PT. VI. 



25 



