8—10. QUANTITIES WHICH CAN ”BE CALCULATED 169
where io is a constant, e = 2.718,' VT is the retarding potential
,
across the positive ion sheath, 6 is the electronic charge, It is Boltzmann’s constant and 7; is the equivalent temperature of the electrons in degrees absolute. If V is the applied potential and K
-
the potential of the gas, With respect to the anode, then
.VT=V_VS(8—3)
Whether such a velocity distribution exists or not may be
.
tested by plotting logarithms of the electron current (i) against V,
as in gure 8—11. Substituting équation 8—3 in 8—2 and taking
logarithms of both sides, gives
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log€ z' = constant +
£— V,
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(8—4)
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kT
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since Vs is constant.
The logarithmic plot Will be a Straight line if
the
distribution
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