8—11. CURRENT TO A SMALL CYLÏNDRICAL PROBE 171
8—11.
Current to ‘a Small Cyündcal Probe.—From the
Kinetic Theory, the number of electrons striking unit area each
second is nä/4, where 77 is the number of electrons per. cubic centimeter, of average velocity, 5. This assumes that the electrons
have a Maxwellian distribution of velocitieS. If each electron carries a charge 5 and the area of the collectingsurface is  -F, then
the current to the probe is ‘ ”'
.
_
'
'
'
_
neäÂF
"
z =
T', ,
‘
{8—10}
where  is the surface area of the probe and F is a correction factor
for the extension of its electrical eld into the surrounding space.
2% = 2wrl, where r is theradius ofthe cylihdrical probe and ! is
_
its length in centimeters. Eor accelerating elds (Va = V —.— 273),
between the gas and the probe, and when Vae/ÆT > 2) as indicated
'
on the right side of
gure ’8f10, it _can‘ be Showri that
‘
.
'
2
Va€
.
“
'
‘
_
F_\/Îr\/ÆT+1
(811)
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_Substitüting in equation8—10 gives = ÏË
-
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_'
‘
"
_
'_
_
_
_‘
=a+s/7, (8—12)
,
_
where cz
_
‘
then', valuesof z2areplottedas j_f
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8—12uanütæsWhthanBeCalculatedfmmthezz—V
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Curve—TheelectrandenSItY<”)canecalculatefromthe
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