268
LIU AND WHITBY
there is a Small difference between the measured transit time and the
true transit time of the ions. The difference is apparently the result of
such factors as the diffusion of ions and the space charge created by
the ions.
—
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MEASUREMENT OF PARTICLE CHARGE AND MOBILITY
The charge spectrometer shown in Fig. 3 was used to measure the
electrical charge on particles. The stream of charged particles was
brought into the charge spectrometer through the annulus opening near
the circumference of the outer cylinder and flowed with the clean air
brought in through the center as a laminar stream. A negative voltage
(for positively charged particles) was applied to the center rod, and the
radial electric field thus established between the center rod and the
outer cylinder forced the charged particles to move toward the rod as
they flowed down with the laminar air stream. Electron-microscope
grids were placed at l-in. intervals along the center rod to collect the
particles. The electrical charge on a particle collected on individual
grids was then calculated from the position ofthe grid and the operating
conditions in the charger and the measured particle size. The laminar
nature of the flow in the charge spectrometer was established by placing
a filter in the filter holder and observing the pattern of dye deposits on
the filter. When the flow was laminar and no voltage was applied to the
center rod, a sharply defined ring was observed.
If a uniform velocity profile for the air stream in the charge
spectrometer is assumed, the mobility of a particle easily can be
shown to be related to the spectrometer dimensions on operating conditions by the following equation:
Zp
where
_ 4 In(ro/r;) ry2 — 4}42
27xV
rj—ri
(5)
Zp = electrical mobility of particle, cm?/statvolt-sec
q = total volumetric rate of flow, cm’/sec
x = distance between point on rod where particle is collected
and entrance, cm
V = voltage applied to center rod, statvolts
Y,, Y2, and r3; = radii shownin Fig. 3
ry = '/ (ro + r3)
For calculation of the charge on the particle, Stokes’ law with Cunning-
ham’s correction must then be used:
Np
_ 37 14pZ>p
Ce
(6)