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Operation, Maintenance, Troubleshooting and Repair

Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

Chapter Seven: The High-Voltage System (continued)

1. 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 7-4A (below) shows the uniform and direct movement of the electrons in an electric field, from the negative cathode to the positive anode, with no magnetic field present.

2. 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.

7.3.1 Effect of the Magnetic Field

In Figure 7-4B two permanent magnets are added above and below the tube structure. In Figure 7-4C, assume the upper magnet is a North Pole and you are viewing from that position. The lower, South Pole magnet, is located underneath the Figure so that the magnetic field appears to be going right through the paper. Just as electrons flowing through a solid wire cause a magnetic field to build up around the wire, so an electron moving through space tends to build up a magnetic field around itself. On one side (left) of the electronís path, this self-induced magnetic field adds to the permanent magnetic field surrounding it. On the other side (right) of its path, it has the opposite effect of subtracting from the permanent magnetic field. The magnetic field on the right side is therefore weakened, and the electronís trajectory bends in that direction, resulting in a circular motion of travel to the anode.

The process begins with a low voltage being applied to the filament which causes it to heat up (filament voltage is usually 3 to 4 VAC, depending on the make and model). Remember, in a magnetron, the filament is also the cathode. The temperature rise causes increased molecular activity within the cathode to the extent that it begins to "boil" off or emit electrons. Electrons leaving the surface of a heated filament wire might be compared to molecules that leave the surface of boiling water in the form of steam. The electrons, however, do not evaporate. They float just off the surface of the cathode, waiting for some momentum.

Electrons, being negative charges, are strongly repelled by like negative charges. So this floating cloud of electrons would be repelled away from a negatively charged cathode. The distance and velocity of their travel would increase with the intensity of the applied negative charge. (Continued on next page)

Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide