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Direct and Indirect Heating of Cathodes

Author: E.E. Kimberly

If a cathode is heated by passing a current through it, it is said to be directly heated. The cathode is then a filament of ribbon or wire. Defects of this construction are: The filament is fragile, and the two ends of the filament are at different Cathode Temperatures

T1 T2 and T3

potentials with respect to the anode. This difference of potential leads to difficulties when the filament is heated by alternating current.

An alternate construction is one in which the coated cathode is in the form of a thin-walled coated tube. Inside the tube is the heating filament, and the cathode is heated by radiation from it. These cathodes require more time to become heated to operating temperature but have the virtues of being rugged and of having no direct connection to the heating element.

Last Update: 2010-10-05