Mr Wilson, On a Portable Gold-leaf Electrometer, etc. 189 



Application of the Instrument to Measurements in Atmospheric 



Electricity. 



Let us fix a horizontal conducting plate, a few square cms. in 

 area, by means of a vertical rod to the terminal of the electro- 

 meter. Let it be surrounded by a considerably large plate lying 

 in the same plane and forming a guard ring for it. On this guard 

 plate place a conducting cover, of which the roof is some cms. 

 above the flat plate. Let the electrometer be momentarily earthed 

 by means of the earthing rod and let the cover be then removed. 

 If this operation has been performed in an open field, the electro- 

 meter will, under normal fine weather conditions, have its potential 

 considerably raised, and the gold leaf will go out of the field of 

 view. The potential can be at once brought back to zero by 

 sliding in the tube of the condenser as described above. The 

 testing plate being now at zero potential, the charge on its ex- 

 posed surface is the same as if it were earth connected. This 

 charge is given at once by the reading of the sliding condenser 

 if it has been previously standardised. The charge per unit area 

 of the plate will be proportional to the strength of the earth's 

 field. 



Let us maintain the potential at zero for some minutes, com- 

 pensating for any change in the earth's field by the proper move- 

 ment of the slider. Then let us replace the cover and draw out 

 the condenser tube to its standard position. The rise of potential 

 indicated by the gold leaf multiplied by the capacity of the system 

 gives the charge gained by the plate in a known number of 

 minutes when kept at zero potential, that is, under the same con- 

 ditions as when earth connected. 



We have thus obtained by these observations a measurement 

 of the charge on our horizontal plate when earth connected and 

 of the current through its surface. The ideal condition would 

 of course be to have the plate on a level with the surface of 

 the ground. We would then have both the charge per unit 

 area of the earth's surface = a (and therefore also the magnitude 

 of the electric field = 4nr<r) and also the vertical current per 

 square cm. of the earth's surface at the place of observation. 



It should be noticed that the earth-air current, as obtained by 

 the above method, includes not only the current carried by ions 

 under the action of the electric field, but also any charge brought 

 down by falling dust particles or rain drops, but not by any current 

 that might be carried by corpuscles with sufficient penetrating 

 power to traverse the testing plate unabsorbed. 



