A heat switch 570 is disclosed as well as a gradiometer having the heat switch. The heat switch is formed from a non-magnetic material such as a semi-conducting material. The semi-conducting material may be provided by way of a Hall effect device. The heat switches are arranged in the gradiometer on a circuit board 856. The circuit board 850 has conducting strips 856 which are connected to conducting strips on a sensor 71 by bridges 852. The heat switch 570 is connected on the opposite side of the circuit board 850 to the strips 856 and processing circuitry 859. A copper substrate 865 is provided on the same side of the circuit board as the heat switch 570 to conduct heat away from the heat switch 570 when the heat switch is closed.