ions in one second of
the electrometer attached to D, hence the observations of this
instrument give 1 - ut/l. If we know the velocity of the gas and the
length of the plates A and B, we can determine t, and since l can be
easily measured, we can find u, the velocity of the positive ion in a
field of given strength. By charging A and C negatively instead of
positively we can arrive at the velocity of the negative ion. In
practice it is more convenient to use cylindrical tubes with coaxial
wires instead of the systems of parallel plates, though in this case
the calculation of the velocity of the ions from the observations is a
little more complicated, inasmuch as the electric field is not uniform
between the tubes.

[Illustration: FIG. 10.]

A method which gives very accurate results, though it is only
applicable in certain cases, is the one used by Rutherford to measure
the velocity of the negative ions produced close to a metal plate by
the incidence on the plate of ultra-violet light. The principle of the
method is as follows:--AB (fig. 10) is an insulated horizontal plate
of well-polished zinc, which can be moved vertically up and down by
means of a screw; it is connected with one pair of quadrants of an
electrometer, the other pair of quadrants being put to earth. CD is a
base-plate with a hole EF in it; this hole is covered with fine wire
gauze, through which ultra-violet light passes and falls on the plate
AB. The plate CD is connected with an alternating current dynamo,
which produces a simply-periodic potential difference between AB and
CD, the other pole being put to earth. Suppose that at any instant the
plate CD is at a higher potential than AB, then the negative ions from
AB will move towards CD, and will continue to do so as long as the
potential of CD is higher than that of AB. If, however, the potential
difference changes sign before the negative ions reach CD, these ions
will go back to AB. Thus AB will not lose any negative charge unless
the distance between the plates AB and CD is less than the distance
traversed by the negative ion during the time the potential of CD is
higher than that of AB. By altering the distance between the plates
until CD just begins to lose a negative charge, we find the velocity
of the negative ion under unit electromotive intensity. For suppose
the difference of potential between AB and CD is equal to a