74 METHODS OF MEASUREMENT [cH. 
The case consists of a rectangular brass box 4 cms. x 4 ems. 
x3cms. A narrow gold-leaf L is attached to a rod R passing 
through a clean sulphur cork. Opposite the gold-leaf is fixed an 
insulated brass plate P, placed about 1 mm. from the wall of the 
box. The movement of the gold-leaf is observed through two 
small windows by means of a microscope provided with a micrometer 
scale. The plate P is maintained at a constant potential (generally 
about 200 volts). The electrometer case is placed in an inclined 
position as shown in the figure, the angle of inclination and the 
potential of the plate beg adjusted to give the desired sensitive- 
ness. The gold-leaf is initially connected to the case, and the 
microscope adjusted so that the gold-leaf is seen in the centre of 
the scale. For a given potential of the plate, the sensitiveness 
depends on the angle of tilt of the case. There is a certain critical 
inclination below which the gold-leaf is unstable. The most 
sensitive position lies just above the critical angle. In a particular 
experiment Wilson found that with an angle of tilt of 80° and with 
the plate at a constant potential of 207 volts, the gold-leaf, when 
raised to a potential of one volt above the case, moved over 200 
scale divisions of the eyepiece, 54 divisions corresponding to one 
millimetre. 
In use, the rod R is connected with the external insulated 
system whose rise or fall of potential 1s to be measured. On 
account of the small capacity of the system and the large movement 
of the gold-leaf for a small difference of potential, the electroscope 
is able to measure extraordinarily minute currents. The apparatus 
is portable. If the plate P is connected to one pole of a dry pile 
the gold-leaf is stretched out towards the plate, and in this position 
can be carried without risk of injury. 
54. Electrometers. Although the electroscope can be used 
with advantage in special cases, it is limited in its application. 
The most generally convenient apparatus for measurement of 
ionization currents through gases is one of the numerous types of 
quadrant electrometers. With the use of auxiliary capacities, the 
electrometer can be used to measure currents with accuracy over 
a wide range, and can be employed for practically every kind of 
measurement required in radio-activity. 
The elementary theory of the symmetrical quadrant electrometer 

