A multichannel far infrared interferometer used on the Joint European Torus (JET) is described. The light source is a 195μm DCN laser. The instrument is of the Mach-Zehnder type, with a heterodyne detection system. The modulation frequency (100kHz) is produced by diffraction from a rotating grating. There are six vertical and two oblique channels. The latter rely on retroreflection from mirrors mounted on the vessel wall. Their vibration is compensated by a second wavelength interferometer at 118.8μm. The various subsystems are described, with emphasis on features necessitated by (a) large path lengths, (b) remote handling requirements, (c) fluctuations in atmospheric humidity, and (d) unmanned automatic operation. Typical measurements, along with real-time and off-line data analysis, are presented. The phase shift measurement is made with an accuracy of 1/20 of a fringe, corresponding to a line integrated electron density of 5x1017 m-2. Comparison with other electron density diagnostics are shown. The introduction of additional optics allows measurements of the Faraday effect and a determination of the poloidal magnetic field distribution. The signal processing and data analysis are described. Errors introduced by the calibration procedure, birefringence of the probing beams, toroidal field pickup, the flux geometry, and the density profile are considered. The Faraday angle is measured with an accuracy of 5% and a time resolution of 1-10ms. The poloidal magnetic field is deduced with an accuracy of ±15%.