This type measures magnetic fields in the shaft surface, the distortion of these fields gives and indication of the torque. The principle behind this is that in some ferromagnetic materials reluctance ( magnetic resistance) is less along the plane of stress than across it. In the torductor three rings are fitted around the shaft each ring having four electromagnetic poles. The centre ring acts as a transformer primary with the two outer secondary rings having their poles arranged 45' apart to the primary poles but in line with each other. The poles are held in a frame so that there is no contact between the poles and the shaft which have a gap of about 3 mm between them.
An alternating current is fed to the centre ring thus generating a magnetic field. This educes an emf in the outer two rings. The outer two ring coils are connected in series in such away that at zero stress the emf generated in each ring is opposite and equal in value giving an output of zero volts.
When torque is applied to the shaft the sress lines are distorted to to the axial. The distorion of this field affects the emf induced in the coils increasing on one side and reducing on the other. Thus a resultant emf of a few milivolts is available at the output. The size of the output is proportional to the stress applied.
