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LIU AND WHITBY
u in diameter will be uncharged under Boltzmann equilibrium. However,
the fraction of particles which are neutral decreases steadily
with increasing particle diameter. For instance, at 0.1 u 42.4% of the
particles are neutral, whereas at 1 yp only 13.3% are neutral. Therefore the aerosol stream was passed between the parallel-plate con-
denser as shown in Fig.
1 to ensure that all particles entering the
particle charger were neutral or, at most, carried a negligible electrical charge. The voltage applied to the condenser plates was such
that all singly charged particles smaller than approximately 0.5 yp
were removed from the aerosol stream and that, for all practical
purposes, the aerosols emerging from the parallel-plate condenser
could be considered as neutral,
AEROSOL CHARGER AND SMALL !ION-MOBILITY ANALYZER
The aerosol charger and small ion-mobility analyzer were combined into one unit as shown in Fig. 2 so that both could be operated
under the same ambient conditions and with the same source of ions in
both devices, This ensures that the mobility of ions measured by means
of the mobility analyzer is actually that of the ions used in charging the
particles. Since it is known!’ that the mobility of air ions is affected by
impurities that may be present in the air and by the age of the ions and
Since it is impractical in these charging experiments to purify the air
stream sufficiently to allow any control of ion mobility, it is essential
that the ion mobility be measured under actual operating conditions.
The principle of operation of the aerosol charger is similar to that
described by Hewitt.! The charging region through which aerosols flow
is bounded on one side by a metal screen and on the other side by a
metal plate. The high voltage applied to the metal plate was in the form
of a 500 cycle/sec square wave. Whenthepolarity of the voltage applied
to the plate was opposite to that of the ions produced by the corona
wires, the ions were drawn into the charging region to charge the
particles. A square wave was used to prevent the precipitation of the
aerosols in the charging region after they became charged. All data
presented here were obtained with a positive voltage applied to the
corona wires, and hence positive ions were used in charging the
particles,
The mobility of small ions at various pressures is measured by
measuring the transit time of the ions in flowing across a drift space
under a known voltage-potential gradient. The mobility is then calculated from the equation
(4)
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