The RM-201 is a "Permatron" controlled rectifier, of
triode configuration with argon fill. The output voltage
can be controlled either by a magnet attached to the
bulb about 1/3 of the way up from the base, or by
an internal "collector" electrode acting as a grid.
Filament uses 2.5 V @ 3.5 A. Peak reverse voltage
is 350; output current is 100 mA.
There was a larger Permatron, the RM-208, which is
basically an 866A mercury rectifier with collector
added, and carrying 866A ratings. An article in
"Electronics," April 1939, covers these tubes.
