The general theory of transversely magnetised ferrite in rectangular waveguide is given in Chapter 6. In particular, Fig. 6.1 shows the planes where the magnetic field in the rectangular waveguide is circularly polarised in a plane parallel to the broad face of the waveguide. However, transversely magnetised ferrite devices are not the only ones to give satisfactory performance in rectangular waveguide. It is also possible to provide variable phase change by using a longitudinally magnetised ferrite rod of appropriate diameter at the centre of rectangular waveguide. It appears that some kind of circular waveguide mode is generated in the ferrite which ought to experience Faraday rotation. With the constraint of the rectangular waveguide walls, rotation cannot occur and a large phase change takes place instead. The longitudinally magnetised phase changer is described in Section 7.7.