ABSTRACT
All substances irradiated by an electromagnetic radiation of sufficiently high frequency are ionized, which means that they lose some of their electrons. For atoms and molecules, there are a number of values for the ionization energy corresponding to the successive loss of a number of electrons. The photoelectric threshold for zinc lies in the ultraviolet. Various spectral radiations, emitted by a mercury vapour lamp and isolated by filtering or by dispersion, are made to fall on a photoelectric cell of a particular type. The cessation of the photoelectric current can be precisely determined only by a very sensitive instrument, such as a mirror galvanometer associated with an electronic tube to increase the sensitivity. Vision and photography which are the two most important detectors of radiation, are photo-chemical phenomena.
