Current and on the Origin of Atmospheric Electricity. 369 



change in the potential of the compensator tube, if not allowed 

 for, would under the conditions of the experiments introduce a 

 corresponding error into the measurement of both the charge on 

 the test-plate and the current into it. It is however easy to test 

 the constancy of the compensator constant from time to time by 

 measuring the displacement required to cause a given change 

 of potential of the gold-leaf system, the cover being on so that 

 the gold-leaf system may have a definite capacity. In the instru- 

 ment actually used there was rarely any sensible change during a 

 series of observations. 



To standardise the compensator the following method was 

 used. A circular brass plate (Fig. 2), " the condenser plate," of 

 the same diameter as the guard ring surrounding the test-plate, 

 was supported by means of three insulating feet on the guard ring. 

 The distance of the condenser plate from the guard ring and test- 

 plate was 128 cms. The condenser plate could at will be earthed 



Fig. 2. 



or connected to the positive terminal of a battery of one or more 

 secondary cells of which the other terminal was connected to the 

 electrometer stand. The compensator was first drawn out to its 

 full extent, the test-plate and condenser plate both being earthed. 

 The earth connections to both plates were now broken, and the 

 condenser plate was brought to a definite potential by means of 

 the cells. This raised the potential of the test-plate, causing a 

 corresponding displacement of the gold-leaf. The compensator 

 was then pushed in sufficiently to bring the potential of the test- 

 plate as indicated by the gold-leaf back to zero. We now have 

 the test-plate at the same potential, zero, as the guard ring, the 

 condenser plate above it being also at a known potential V; we 

 can thus readil} 7 calculate the charge on the test-plate. An equal 

 and opposite charge has been removed from the system by the 

 displacement of the compensator. By repeating this process for 

 various values of the potential of the condenser plate the charge 

 held on the compensator rod corresponding to any position of the 

 compensator can be determined, and thus a calibration curve for 



