374 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
narrow strip of metal with a gold-leaf attached. The strip of 
metal was fixed to the upper rod by means of a small sulphur bead. 
The upper rod was connected to a sulphur condenser with an 
Exner electroscope B attached to indicate its potential. The 
gold-leaf system was initially charged to the same potential as 
the upper rod and condenser by means of a fine steel wire which 
was caused to touch the gold-leaf system by the attraction of a 
magnet brought near it. The rate of movement of the gold-leaf 
IF 
Condenser 

Earth 

Earth 
Fig. 60. 
was measured by means of a microscope provided with a micro- 
meter eye-piece. By keeping the upper rod at a slightly higher 
potential than the gold-leaf system, it was ensured that the loss 
of charge of the gold-leaf system was not in any way due to a 
conduction leakage across the sulphur bead. 
The method employed by Wilson in these experiments is 
very certain and convenient when an extremely small rate of 
discharge is to be observed. In this respect the electroscope is 
able to measure with certainty a rate of loss of charge much 
smaller than can be measured by a sensitive electrometer. 

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