A large detector for cosmic magnetic monopoles has been designed, tested and used for over a year. It contains three independent superconducting loops, whose currents are monitored by three RF SQUID sensors. Two of the loops, each of diameter 170 mm, are of gradiometric design; the third loop, 950 mm high and 115 mm wide, maximises the available detector area, but responds with a non-unique signal to a monopole. The detector assembly is shielded from the ambient magnetic field by a set of high-permeability room-temperature shields and a thick superconducting shield. Extensive precautions were taken to exclude mechanical and electrical interference. A computerised data acquisition system measures the detector loop currents 20 times per second, and simultaneously records the outputs of monitoring devices for vibration, RF interference, magnetic field and cryostat helium pressure. The detector has proved highly satisfactory, with an active observation time of over 90% of the running time.