A magnetic field sensor for determining the strength and/or direction of a component of an ambient magnetic field, comprising electrical circuitry which incorporates: a magnetic sensing element comprising a coil surrounding magnetic material, a voltmeter connected in the circuitry to provide a measure of the voltage across the magnetic sensing element, an electrical energy source connector connected to the magnetic sensing element to supply an electrical current which passes through that element when the sensor is in use, and switches (S 3, S 4, S 5 and S 6) to reverse the direction of the flow of electrical current through the magnetic sensing element between successive measurements of the said voltage by the said voltmeter, the electrical energy source connector and the said switches (S 3, S 4, S 5 and S 6) being connected to the said coil, wherein the voltmeter is connected to measure the voltage across the said coil, and the circuitry further comprises a timer connected within the circuitry to cause the voltmeter to provide measurements V 1 and V 2 respectively at times T 1 and T 2 which occur during saturation of the magnetic material after successive operations of the switches (S 3, S 4, S 5 and S 6) respectively at times T 0 and T 5, to initiate the passage of electrical current through the coil in respective opposite directions, such that T 1 - T 0 = T 2 - T 5 = ΔT, where ΔT is a predetermined period of time, and a processor is connected in the circuitry to provide an indication of the value of V 1 -V 2, the magnitude of which provides an indication of the strength of the said component, and the sign of which provides an indication of the direction of the said component. a magnetic sensing element comprising a coil surrounding magnetic material, a voltmeter connected in the circuitry to provide a measure of the voltage across the magnetic sensing element, an electrical energy source connector connected to the magnetic sensing element to supply an electrical current which passes through that element when the sensor is in use, and switches (S 3, S 4, S 5 and S 6) to reverse the direction of the flow of electrical current through the magnetic sensing element between successive measurements of the said voltage by the said voltmeter, the electrical energy source connector and the said switches (S 3, S 4, S 5 and S 6) being connected to the said coil, wherein the voltmeter is connected to measure the voltage across the said coil, and the circuitry further comprises a timer connected within the circuitry to cause the voltmeter to provide measurements V 1 and V 2 respectively at times T 1 and T 2 which occur during saturation of the magnetic material after successive operations of the switches (S 3, S 4, S 5 and S 6) respectively at times T 0 and T 5, to initiate the passage of electrical current through the coil in respective opposite directions, such that T 1 - T 0 = T 2 - T 5 = ΔT, where ΔT is a predetermined period of time, and a processor is connected in the circuitry to provide an indication of the value of V 1 -V 2, the magnitude of which provides an indication of the strength of the said component, and the sign of which provides an indication of the direction of the said component.