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Ralph Mowery

In article ,
says...

Actually, they ARE heat sinks. Selenium rectifiers were much less efficient
than silicon, or even germanuim rectifiers, with a forward voltage drop of
about 1V per cell (plate). That means that if your rectifier has 16 cells,
then the rectifier, as a whole, will have AT LEAST 16 volts of forward
voltage drop. Multiply that by the current through the rectifier, and you
have the number of watts that has to be dissipated, thus, the need for the
heat sink plates.

Yes, the actual plates are heat sinks, but I was loose with my words and
just using that as a refference as to how many sections there are. That
is each plate coresponds to a diode type junction in series. So if you
count 5 plates it is similar to having 5 dioides in series. Each one
good for about 25 volts of inverse voltage and dropping about a volt.
That would make around 5 volts forward drop and good for about 125 piv.

Using the term plate was to me just a quick way of estimating how much
voltage drop and PIV the stack could handle.