it] METHODS OF MEASUREMENT 75 
as given in the text-books is very imperfect. It is deduced that 
the sensibility of the electrometer—measured by the deflection of 
the needle for 1 volt p.p. between the quadrants—varies directly 
as the potential of the charged needle, provided that this potential 
is high compared with the p.p. between the quadrants. In most 
electrometers however, the sensibility rises to a maximum, and then 
decreases with increase of potential of the needle. For electrometers 
in which the needle lies close to the quadrants, this maximum 
sensibility 1s obtained for a comparatively low potential of the 
needle. A theory of the quadrant electrometer, accounting for this 
action, has been recently given by G. W. Walker. The effect 
appears to be due to the presence of the air space that necessarily 
exists between adjoming quadrants. 
Suppose that it is required to measure with an electrometer 
the ionization current between two 
horizontal metal plates A and B 
(Fig. 13) on the lower of which some 
active material has been spread. If 
the saburavion current AS required, ‘Lk 
the insulated plate A is connected 


with one pole of a battery of sufficient ik 
EM.F. to produce saturation, the B 
other pole being connected to earth. pag Active Material 
The insulated plate B is connected i Sacre 
AG poennas |i|t\—~ Zartn 
with one pair of quadrants of the 
electrometer, the other pair being Tate, 18 
earthed. By means of a suitable key 
K, the plate B and the pair of quadrants connected with it may be 
either insulated or connected with earth. When a measurement 
is to be taken the earth connection is broken. If the positive pole 
of the battery is connected with A, the plate B and the electro- 
meter connections immediately begin to be charged positively, and 
the potential, if allowed, will steadily rise until it is very nearly 
equal to the potential of A. As soon as the potential of the 
electrometer system begins to rise, the electrometer needle com- 
mences to move at a uniform rate. Observations of the angular 
movement of the needle are made either by the telescope and scale 
1 Phil. Mag. Aug. 1903. | 
