The equipment in the track consists of a protective ramp preceding a Permanent Inductor (AWS3) and an Electro Inductor (AWS1).
The Permanent Inductor has its North Pole uppermost and this magnetic field by itself gives a “caution” indication to the driver. The Electro Inductor (AWS1), when energised, has its South Pole uppermost and this South Pole following the North Pole of the permanent inductor gives a “Clear” indication to the driver. On bi-directional lines, special permanent inductors called Suppressor Inductors(AWS2) may be used to prevent a train receiving A.W.S. indications at a signal for the opposite direction to which the train is travelling. These inductors have a permanent inductor and, in addition, a suppressing coil which, when energised, diverts the magnetic flux, suppressing any indication to the train.
For detecting the magnetic fields produced by the AWS track-mounted units, please see MRD’s MagSense.