Abstract
Field dependent chain formation in ferromagnetic colloids is discussed. It is shown that chains form if the density of ferromagnetic particles is sufficiently low and if the dipole-dipole coupling is sufficiently strong. In small magnetic fields the chains may only be metastable ; they are stable in high fields. General formulae for calculating the mean chain length as a function of density, dipole-dipole coupling constant and dipole-field coupling constant are given and compared with available experimental data.