a function that characterizes the distribution of the electric charge or magnetic moment within some microsystem, such as an atom or atomic nucleus, or within an elementary particle. The form factor of an atom is determined by the distribution of atomic electrons; the form factor of a nucleus is determined primarily by the distribution of nucleons (protons and neutrons) within the nucleus. In the latter case, virtual mesons also contribute to the form factor; they are the basis for the nucleon exchange that gives rise to nuclear forces. The existence of a form factor has also been established for strongly interacting elementary particles—hadrons, such as nucleons and pions. Experiments on the elastic scattering of high-energy electrons and muons by hadrons lead to the conclusion that the size of the region over which the electric charges and magnetic moments of the hadrons are distributed is of the order of 10^–13 cm. These data directly attest to the complex structure of hadrons. At the same time, experiments indicate that the maximum range at which the charge of an electron or muon is appreciable does not exceed 10^–15cm.

The form factor of hadrons that characterizes the dimensions of hadrons in processes involving the weak interaction is postulated by analogy with form factors characterizing the distribution of magnetic moment.