" Z " was the number known as a physical characteristic of atoms after what year ?
Following conversations in 1913 with Niels Bohr , a fellow worker in Ernest Rutherford 's Cavendish laboratory , Moseley had become interested in the Bohr model of the atom , in which the spectra of light emitted by atoms is proportional to the square of Z , the charge on their nucleus ( which had just been discovered two years before ) .