negative ions to move under the electric
field through a distance equal to AB, the distance between the planes.
Then if T is greater than T2 all the ions will have been driven from
between the plates before the field is reversed, and therefore the
positive charge received by B will not depend upon T. Next let T be
less than T2 but greater than T1; then at the time when the field is
reversed all the negative ions will have been driven from between the
plates, so that the positive charge received by B will not be
neutralized by the arrival of fresh ions coming to it after the
reversal of the field. The number of positive ions driven against the
plate B will be proportional to T. Thus if we measure the value of the
positive charge on B for a series of values of T, each value being
less than the preceding, we shall find that until T reaches a certain
value the charge remains constant, but as soon as we reduce the time
below this value the charge diminishes. The value of T when the
diminution in the field begins is T2, the time taken for a positive
ion to cross from A to B under the electric field; thus from T2 we can
calculate the velocity of the positive ion in this field. If we still
further diminish T, we shall find that we reach a value when the
diminution of the positive charge on B with the time suddenly becomes
much more rapid; this change occurs when T falls below T1 the time
taken for the negative ions to go from one plate to the other, for now
when the field is reversed there are still some negative ions left
between the plates, and these will be driven against B and rob it of
some of the positive charge it had acquired before the field was
reversed. By observing the time when the increase in the rate of
diminution of the positive charge with the time suddenly sets in we
can determine T1, and hence the velocity of the negative ions.

The velocity of the ions produced by the discharge of electricity from
a fine point was determined by Chattock by an entirely different
method. In this case the electric field is so strong and the velocity
of the ion so great that the preceding methods are not applicable.
Suppose P represents a vertical needle discharging electricity into
air, consider the force acting on the ions included between two
horizontal planes A, B. If P is the density of the electrification,
and Z the vertical component of the electric inte