i = ------
l
where
N = number of ions present in the gas,
e = charge on each ion,
u = sum of the velocities of the positive and negative ions.
Since the value of N is the same as the number of drops, and the velocity u is known, the value of e can be determined.
In his last determination J. J. Thomson found that
e = 3·4 × 10-10 electrostatic units.
A very concordant value, namely, 3·1 × 10-10, has been obtained by H. A. Wilson[[70]], by using a modified method of counting the drops. A check on the size of the drops, determined by their rate of fall, was made by observing the rate at which the drops moved in a strong electric field, arranged so as to act with or against gravity.