ll drive part of the particles against the insulated plate. This
proves that the particles in question are positively electrified.
The amount of the charge which they carry may be measured by the
electrometer.

The aggregate amount of the electrical charge carried by these minute
particles in the gas being known, it is obvious that could we know the
number of particles involved the simplest calculation would determine
the charge of each particle. Professor Thompson devised a singularly
ingenious method of determining this number. The method was based on
the fact discovered by C. T. R. Wilson that charged particles acted as
nuclei round which small drops of water condense much as dust particles
serve the same purpose. "In dust-free air," says Professor Thompson,
"as Aitken showed, it is very difficult to get a fog when damp air is
cooled, since there are no nuclei for the drops to condense round. If
there are charged particles in dust-free air, however, the fog will be
deposited round these by super-saturation far less than that required to
produce any appreciable fog when no charged particles are present.

"Thus, in sufficiently supersaturated damp air a cloud is deposited on
these charged particles and they are thus rendered visible. This is the
first step towards counting them. The drops are, however, far too small
and too numerous to be counted directly. We can, however, get their
number indirectly as follows: suppose we have a number of these
particles in dust-free air in a closed vessel, the air being saturated
with water-vapor; suppose now that we produce a sudden expansion of the
air in the vessel; this will cool the air, it will be supersaturated
with vapor, and drops will be deposited round the charged particles. Now
if we know the amount of expansion produced we can calculate the cooling
of the gas, and, therefore, the amount of water deposited. Thus we know
the volume of water in the form of drops, so that if we know the volume
of one drop we can deduce the number of drops. To find the size of a
drop, we make use of the investigations made by Sir George Stokes on the
rate at which small spheres fall through the air. In consequence of
the viscosity of the air small bodies fall exceedingly slowly, and the
smaller they are the slower they fall." *

Professor Thompson gives us the formula by which Stokes made his
calculation. It is a relatively simple algebraic one, but need not be
repeated here. For us it suff