Hot Cathode - Principles

Principles

Hot cathodes may be either directly heated, where the filament itself is the source of electrons, or indirectly heated, where the filament is electrically insulated from the cathode; this configuration minimizes the introduction of hum when the filament is energized with alternating current. The filament is most often made of tungsten. With indirectly heated cathodes, the filament is usually called the heater instead. The cathode for indirect heating is usually realized as a nickel tube which surrounds the heater.

The first cathodes consisted simply of a tungsten filament heated to white incandescence (known as bright emitters). Later cathodes are typically covered with an emissive layer, made of a material with lower work function, which emits electrons more easily than bare tungsten metal, reducing the necessary temperature and lowering the emission of metal ions. Cathodes can be made of pure sintered tungsten as well; tungsten cathodes in the shape of a parabolic mirror are used in electron beam furnaces. Thorium can be added to tungsten to increase its emissivity, due to its lower work function. Some cathodes are made of tantalum.

Read more about this topic:  Hot Cathode

Famous quotes containing the word principles:

    Ah, I fancy it is just the same with most of what you call your “emancipation.” You have read yourself into a number of new ideas and opinions. You have got a sort of smattering of recent discoveries in various fields—discoveries that seem to overthrow certain principles which have hitherto been held impregnable and unassailable. But all this has only been a matter of intellect, Miss West—superficial acquisition. It has not passed into your blood.
    Henrik Ibsen (1828–1906)

    The mode of founding a college is, commonly, to get up a subscription of dollars and cents, and then, following blindly the principles of a division of labor to its extreme,—a principle which should never be followed but with circumspection,—to call in a contractor who makes this a subject of speculation,... and for these oversights successive generations have to pay.
    Henry David Thoreau (1817–1862)

    In child rearing it would unquestionably be easier if a child were to do something because we say so. The authoritarian method does expedite things, but it does not produce independent functioning. If a child has not mastered the underlying principles of human interactions and merely conforms out of coercion or conditioning, he has no tools to use, no resources to apply in the next situation that confronts him.
    Elaine Heffner (20th century)