June 18, 1941
Dr. James B. Conant
National Defense Research Committee
1530 P Street NW
Dear Dr. Conant:
I was very glad to receive your letter of June 13, since I have plenty of
time to think about things and like to have a problem to work on. I wish to suggest
a method for determining particle size in fogs. So far as I know, the method is new,
and after making some calculations I think that it should be satisfactory. We ought
to be able to build an apparatus which will register with considerable accuracy particle
size and particle-size distribution in a fog such as described in your letter.
The proposed method involves giving an electric charge to the particles or
to a fraction of them and then measuring the rate at which they are attracted to a
condenser plate. The rate at which particles with a given charge are pulled to a condenser
plate varies inversely with the radius of the particle, as shown by Stokes' law. Two
methods of distributing the charge on the particles might be used. The first would
involve placing the same charge on all particles or on a representative fraction of
them. The second would involve charging the particles to extents determined by their
sizes. I think that the second method might be the better one. If electrons or ions
were passed through the fog in such a way that each particle received a charge proportionate
to its effective cross-sectional area, the large particles would then move faster
in the field than the smaller particles. Some study of the method of charging the
particles most effectively should be made.
After the particles had been charged, a stream of the charged fog would be
liberated in the middle of a stream of air moving non-turbulently between the plates
of a condenser, with plate separation perhaps 2 centimeters and field of a few thousand
volts per centimeter. The charged particles would then move toward one plate from
their original level at rates proportional to their charges and inversely proportional
to their radii. They would strike collectors, placed in sequence along one of the
condenser plates. By amplifying the currents flowing from the different collectors
the size and size distribution for the particles in the fog would be found.
I have made various order-of-magnitude calculations which look favorable
to the success of the method. With a few thousand volts between the condenser plates
the Stokes' law speed of the particles is of the order of 1 centimeter per second,
which is reasonable. The current reaching each collector plate is also measurable,
being such that it could easily be measured with a one vacuum tube amplifier, F.P.
54. For field work I would suggest a simple apparatus with three or five collectors
and with variable condenser voltage. Probably one amplifier could be used for all
of the collectors, being shifted along the series to make the readings. For careful
laboratory work an instrument with more collectors might be useful.
If your study of the field has shown that an instrument of this sort has
not been built, I think that it would be worth while to follow up this idea. I could,
I think, arrange to have the work done here.
My address from June 23 to July 18 will be Chemistry Department, University
of Chicago, Chicago Illinois. I am leaving here Friday night, June 20.
With best regards, I am