1434 
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difference of the two ionisation currents is measured which are 
generated in the two chambers. With sufficiently strong currents 
great accuracy can be attained in this way. 
at high potentials of opposite sign. Under these circumstances the 
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It is worth noticing, that the various small imperfections in the 
method of measuring, which are usually unnoticed, become appa- 
rent when the method described above is followed. B. g. when 
each current is measured separately, the saturation appears to be 
complete at a potential of about 500 volts. But when the difference 
between the currents was measured, which was usually under 5°/, 
of each current separately, it was found that the current under 
these circumstances increased with the voltage. When the potential 
