Contemporary Advances in Physics - XI - Ionization

T O N I Z A T I O N , in its most general sense, signifies a segregation of positive from negative charge within the volume of a substance which as a whole is (or initially was) electrically neutral. In practice a gas (for instance) is said to be if charges of either sign can be extracted from it. Charged particles of both signs, electrons and ions, can be drawn out from a gas in which an electrical discharge is being maintained; in such a condition, therefore, a gas is ionized. Millikan's droplets, floating around in a gas which had recently been irradiated, absorbed charges of either sign out of the gas, which therefore was ionized by the radiation and remained ionized for some time afterward. A negatively-charged electrode immersed in a carefully-screened gas receives very little charge from it; this condition continues if the gas is bombarded with electrons having less than a certain speed; let the speed of the bombarding electrons be increased past this limit, and the electrode begins to receive positive charge-- the gas is ionized by the electrons. Dilute electrolytic solutions are evidently in a continual and spontaneous state of ionization.