Mutual inductance highly independent of primary winding position and ambient fields

A new design of a high precision mutual inductor is described. Its main features are: immunity to ambient magnetic and electric fields and independence of mutual inductance on the shape and position of the primary winding. These results are achieved by using an electric screen and a hollow toroidal magnetic shield. To avoid the saturation of the magnetic shield two windings of the same number of turns, connected in series-opposition, are wound, one outside and one inside the magnetic shield.