This will become easier to understand as the description of operation is considered. The antenna is coupled to the waveguide , a hollow metal enclosure, into which the antenna transmits the RF energy. The theory of magnetron operation is based on the motion of electrons under the combined influence of electric and magnetic fields. For the tube to operate, electrons must flow from the cathode to the anode. The force exerted by an electric field on an electron is proportional to the strength of the field.
Electrons tend to move from a point of negative potential toward a positive potential.
Figure 3-A shows the uniform and direct movement of the electrons in an electric field with no magnetic field present, from the negative cathode to the positive anode. The force exerted on an electron in a magnetic field is at right angles to both the field itself, and to the path of the electron. The direction of the force is such that the electron proceeds to the anode in a curve rather than a direct path. In Figure 3-B two permanent magnets are added above and below the tube structure. In Figure 3-C, assume the upper magnet is a north pole and you are viewing from that position.