A magnetic beta‐ray spectrometer of high resolution has been constructed. The theory of the resolution of this instrument has been investigated in detail and applied to the design of the slit system. The effects of scattering within the vacuum chamber have been studied and reduced to a negligible value. The corrections for the efficiency of the Geiger‐Mu¨ller counter and for the absorption in the counter window have been obtained as a function of the momentum of the incoming particles.