Metal rectifier

[1][2] In any case the result is that there is a depletion region in the semiconductor, with a built-in electric field, and this provides the rectifying action.

Compared to later silicon or germanium devices, copper-oxide rectifiers tended to have poor efficiency, and the reverse voltage rating was rarely more than a few volts.

Although they were reasonably efficient in this application, (at least compared to vacuum-tube rectifiers), their internal resistance tended to increase as they aged.

Apart from reducing the available high voltage, this tends to make them run hotter, producing an unpleasant smell as the selenium starts to evaporate.

A layer of selenium was applied to a sheet of soft iron foil, and thousands of tiny discs (typically 2mm diameter) were punched out of this and assembled as "stacks" inside ceramic tubes.