which are cut out as the sleeve rotates as the edges 10 of the cut out portions move past the sensor 9 they serve as markers which disrupt its magnetic field creating an electrical signal 11 in a field coil carried within the device 9 these signals 11 are carried off by wires 12 for processing by a processor 13 because of the mechanics at linkages provided by the drive shaft 3 and cv joints 4 5 the sleeve 7 turns at the same rotational speed as the wheels 1 any measurement made of the rotational speed of the sleeve 7 therefore is a measure of wheel speed in fig4 the signal 11 is processed in the processor 13 which compares the duration between the arrival of pulses from the sensor 9 to an internally established clock rate and provides as an output a signal 17 to display 20 to provide a measure of wheel speed the preferred form of processor operation may be based on detecting three transitions the processor will wait for a first transition before starting to store counts in a zeroed counter the counts can be provided by the clock signal of a simple basic microprocessor the processor continues storing counts until a second transition is encountered it will note the changed state and continue counting until a third transition takes place the.