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



removal by the electric field applied. In the improved methods, 

 as used for example by Gerdien, the air is drawn through a tube 

 with an axial rod, a sufficiently small difference of potential being- 

 maintained between the tube and the rod to ensure that only a 

 very small proportion of the total number of ions is removed by 

 "the field from the air passing through, so that the charge acquired 

 by the rod is proportional to the potential difference, i.e. there is 

 no approach to saturation. 



The present paper contains an account of experiments made to 

 test a method suggested in a paper recently read before this Society 

 (Proc. Camb. Phil. Soc. Vol. xin. p. 184). An insulated conductor 

 connected to an electrometer is initially at zero potential and 

 under a metal cover. The earth connection is broken and the 

 cover removed, the conductor being thus exposed to the earth's 

 electrical field. The potential of the conductor is thus raised, but 

 is at once brought back to zero by means of a compensator. When 

 this adjustment has been made we know that the charge removed 

 from the electrometer and its connections by the displacement of 

 the compensator is equal and opposite to that held on the exposed 

 part of the conductor when at zero potential by the action of the 

 earth's field. When the compensator has once been standardised 

 its readings measure the charge on the exposed conductor when 

 kept at zero potential ; this charge will be the same as if the con- 

 ductor were earth connected. If now by means of the compensator 

 the conductor be maintained at zero potential for a few minutes 

 and the cover be then replaced, the new reading of the compen- 

 sator, when again adjusted to bring the electrometer reading back 

 to its zero, measures the charge which has entered the conductor 

 from the atmosphere in the given time. 



By this method both the charge on an exposed conductor and 

 the current flowing into it from the atmosphere, when it is main- 

 tained under the same conditions as if it were connected to the 

 earth, are directly determined. The method has considerable 

 advantages over the more indirect methods. 



We may be said to determine directly the actual charge which 

 has passed from the atmosphere in a known time through a con- 

 ductor into the earth. Under ideal conditions the conductor would 

 take the form of a flat plate at the level of the earth's surface; we 

 should then obtain directly the current per square cm. of a flat 

 portion of the earth's surface. In practice it is not convenient to 

 proceed in this way, but to measure the current through and 

 charge upon a conductor of suitable form when kept at zero 

 potential, and to assume the ratio of the current to the charge 

 (the dissipation coefficient) to be the same for all exposed con- 

 ductors ; then if the potential gradient and hence the charge per 

 square cm. of the ground be known the earth-air current may be 



