.
7—5.
IONIZING POTENTIALS
,
147
‘
_
A better method of measuring the minimum ioni2ing potential,
originated
Lenard in 1902, involves the use of ‘a three—electrode
_
*
tube.
The circuit is shown in gure 7—7; Here the grid is made
positive so that electrons are accelerated toward it and pass into
the space between grid and plate where they are reversed by asmall
negative potential on the plate and return to the grid. The voltage
of the grid is increased so as to speed up the electrons until they
_
have sufcient energy to ionize the gas betWeen grid and plate.
This is made known by observing the current owing to the plate
_
by means of the galvanometer G. Thus in gure
this
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