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. <EOS>