spiral track 19 is within the magnetic field the magnetic field causes electrical current to flow in the spiral resonator 13 resulting in a change in the magnetic field itself the spiral resonator 13 induced changes in the magnetic field also affect the electrical current in the spiral track 19 changing currents in the spiral track 19 also changes the voltage in the spiral track 19 a peak detector 23 monitors the changing voltage and sends the peak voltage to a gain stage 30 which amplifies it the amplified voltage from the gain stage 30 is then read via an analog to digital converter 31 by the microcontroller 28 the microcontroller 28 uses the measured voltage to calculate the angular position of the spiral resonator 13 in relation to the spiral track 19 and sends that calculation out as the spiral sensor output 29 the spiral sensor output 29 of the spiral resonator can be used as a control signal the control signal can be used to control the brakes of a vehicle such as a train an aspect of the embodiment illustrated in fig9 is the use of a microprocessor other functionally equivalent electrical and electronic circuits can be used instead of a microprocessor an oscillator can supply a signal to the spiral drive circuitry 24 the output of the gain stage 30 can be used directly as a control signal the gain stage 30 output can also be passed through analog or digital conditioning circuitry to produce the sensor.