An equipotential-bus electromagnetic undulator has been developed for a far infrared compact free-electron laser. It has 2 m length, 5.6 mm gap, and 12.5 mm period. The field distribution along the undulator is formed by 2.25 mm thick poles of low-carbon steel with 4 mm thick Nd-Fe-B permanent magnets between them. The amplitude of the magnetic field is up to 6 kG. The variation of current allows us to vary the field amplitude within 10% in a constant gap. For the distribution of the peak amplitude along the undulator we obtained random r.m.s. deviation less than 0.5% without further field correction or individual adjustment of each iron pole or magnet. (C) 1998 Elsevier Science B.V. All rights reserved.