PARTICLE CHARGING AT LOW PRESSURES
273
whether any of these small particles were charged and, if so, what
fraction were charged,
To answer these questions two tests were performed in which
uranine aerosols were used to measure the fraction collected on the
center rod and the afterfilter. The operating conditions in the charger
were identical to those used in obtaining the data shownin tests 35, 36,
and 37 in Fig. 7, the electric-field intensity being 240 volts/cm and the
Nt product 1.05 x 10° ions/cm?® per second. The aerosols were produced
by
an
atomizer—impactor (described in Ref. 9). The geometrical
standard deviation of the particle size distribution was approximately
1.5. The mass median diameters of the aerosols used were 0.049 and
0.028 up, respectively, both of which were smaller than the horizontal
intercept of 0.052 1 as shown in Fig. 7. The charge spectrometer was
operated with a sufficiently high voltage on the center rod so thatall
particles that were charged would be collected by the center rod. The
mass of uranine collected on the center rod and on theafterfilter were
determined separately in a fluorimeter. The fraction of particles that
were charged was then calculated from these data. It was foundthat,
under these charging conditions, 56%of the 0.049-u particles were
charged whereas only 24% of the 0.026- particles were charged.
A theoretical equation is derived in the appendix to this paper for
calculating the fraction of particles that are charged. The experimental
data obtained appear to agree well with the theoretical predictions.
Figure 9 is a plot of the fraction, f, of particles that are charged as a
T
c
dT
i
T
1.0
oO
has
®
—
—
a
= 0.8
oO
'e'
vw
4
Y
-
aa
—
-
4
e 0.6 F—
4
a
rs
—
oO
—
0.4
4
[~
an
Zz
Q
0
<
&
—~
0.2
0
0
1
l
{
}
L
1.0
2.0
3.0
4.0
5.0
Nt
(Nh,
Fig. 9—Fraction of particles charged.
6.0