70 METHODS OF MEASUREMENT [CH. 
material, it is necessary to employ methods for measuring currents 
much smaller than can be conveniently detected by an ordinary 
galvanometer. The most convenient apparatus to employ for this 
purpose is one of the numerous types of quadrant electrometer or 
an electroscope of special design. For many observations, especially 
where the activity of the two substances is to be compared under 
constant conditions, an electroscope offers a very certain and simple 
method of measurement. As an example of a simple apparatus 
of this kind, a brief description will be given of the electroscope 
used by M. and Mme Curie in many of their earlier observations. 
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Fig. 10. 
The connections are clearly seen from Figure 10. The active 
material is placed on a plate laid on top of the fixed circular plate 
P, connected with the case of the mstrument and to earth. The 
upper insulated plate P’ is connected to the insulated gold-leaf 
system LL’, Sis an insulating support and L the gold-leaf. 
The system is first charged to a suitable potential by means of 
the rod C. The rate of movement of the gold-leaf is observed by 
means of a microscope. In comparisons of the activity of two 
specimens, the time taken to pass over a certain number of 
divisions of the micrometer scale in the eyepiece is observed. 
Since the capacity of the charged system is constant, the average 
rate of movement of the gold-leaf is directly proportional to the 
ionization current between P and P’, i.e. to the intensity of the 
