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:
“Magic is akin to science in that it always has a definite aim intimately associated with human instincts, needs, and pursuits. The magic art is directed towards the attainment of practical aims. Like other arts and crafts, it is also governed by a theory, by a system of principles which dictate the manner in which the act has to be performed in order to be effective.”
—Bronislaw Malinowski (1984–1942)
“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)
“[The sceptic] must acknowledge, if he will acknowledge any thing, that all human life must perish, were his principles to prevail. All discourse, all action would immediately cease, and men remain in a total lethargy, till the necessities of nature, unsatisfied, put an end to their miserable existence.”
—David Hume (1711–1776)