The design of a lens spectrometer with respect to economy in power consumption and copper weight, resolving and collecting power has been studied by small scale experiments The construction and performance of a large β-ray spectrometer is described which focuses 15 MeV electrons when its coil is energized by 50 amp at 360 volts. The coil is cooled by a flow of 12 gallons of water per minute Maximum resolution and collecting power of the spectrometer are 0 8% and 0 6% respectively The influences of spherical aberration, source and window size on the resolution of the thick lens spectrometer are discussed.