The ions of a contrary sign may be almost completely separated by placing the ionised gas in a suitably disposed field.

In the neighbourhood of a negative disk there remain hardly any but positive ions, and against a positive disk none but negative; and in effecting a separation of this kind, it will be noticed that condensation by negative ions is easier than by the positive.
It is, consequently, possible to cause condensation on negative centres only, and to study separately the phenomena produced by the two kinds of ions.

It can thus be verified that they really bear charges equal in absolute value, and these charges can even be estimated, since we already know the number of drops.

This estimate can be made, for example, by comparing the speed of the fall of a mist in fields of different values, or, as did J.J.Thomson, by measuring the total quantity of electricity liberated throughout the gas.
At the degree of approximation which such experiments imply, we find that the charge of a drop, and consequently the charge borne by an ion, is sensibly 3.4 x 10^{-10} electrostatic or 1.1 x 10^{-20} electromagnetic units.

This charge is very near that which the study of the phenomena of ordinary electrolysis leads us to attribute to a univalent atom produced by electrolytic dissociation.
Such a coincidence is evidently very striking; but it will not be the only one, for whatever phenomenon be studied it will always appear that the smallest charge we can conceive as isolated is that mentioned.