One of the mysteries of the English language finally explained.
A parameter relating current to voltage in some thermionic diodes (e.g. in an electron gun), equal to IV −3/2, where I is the anode current and V is the anode voltage.
1920s. Perhaps from perv- + -eance, or perhaps from pervi- + -ance, after conductance, permittance, etc., with spelling in -eance after words such as permeance.
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