604 Prof. J. S. Townsend on the Genesis of Ions 



were to earth the capacity of ci was ai)i)roxiinately 60 electro- 

 static units, and c'2 and e- 120 each. The ratios of the capa- 

 cities of the plates were determined accurately by experiment. 

 The rod connected to the movable electrode B was con- 

 nected to the insulated quadrants of a sensitive electrometer 

 which could be joined to any of the plates Cj, C2, C3. At the 

 beginning of an experiment the electrometer is at zero 

 potential and insulated. The potential of the plates d, which 

 is varied during an experiment, is zero at the beginning. 

 While the electrode B is acquiring a positive charge (the 

 electrode A being at a high positive potential) the plates d 

 are brought to negative potentials so as to keep the electro- 

 meter reading approximately at zero during the experiment. 

 At the end of the experiment the potential Y of the plates d 

 required to bring the electrometer reading exactly to zero is 

 found. 



The quantity of electricity Q acquired by the current 

 through the gas is thus 



Vci, Y(ci + C2), or V(ci-|-C2 + C3) 



according to the number of plates c joined to the insulated 

 quadrants. 



This method has many advantages. The capacity of the 

 electrometer need not be determined, and the variation of 

 the capacity of the electrode B does not enter into the 

 measurements. The electrometer being used only as a de- 

 tector no errors are introduced by the unequal values of the 

 readings at the different parts of the scale which are noticeable 

 in most electrometers. In addition the method is useful in 

 cases where it is desirable to maintain a constant force 

 between the electrodes during an experiment, since the 

 potential of the plates d is easily varied so as to keep the 

 electrometer near the zero while the current is passing. 



The potential of the plates d can be easily adjusted by a 

 potentiometer method. A set of 50 equal resistances^ 20 ohms 

 each, were arranged in series, and by means of a sliding con- 

 tact it was possible to connect the plates with any of the 

 junctions of the 20-ohm coils. The two terminals of the 

 1000 ohm resistance were joined to a battery of known elec- 

 tromotive force E, the positive terminal being connected to 

 earth. 



The value of V required to bring the electrometer to zero 

 can be estimated accurately by observing the small deflexions 

 on opposite sides of the zero when the plates are in connexion 

 with two consecutive junctions of the resistances. 



