A measurement device (10) comprising a sensing circuit (30) which is electrically floating and fully isolated. The sensing circuit (30) has a first (134) and a second (148) transformer across which is connected a measurement circuit (110). The device is so constructed that any signal generated in the coil (132) of the said first transformer (134) that is connected to the measurement circuit (110) caused by a change in any external magnetic field relative to the measurement circuit (110) is in anti-phase with the corresponding signal generated in the coil (164) of the said second transformer (148) that is connected to the measurement circuit (110).