405 line Nipkow Discs

[beamcurrent]

RCA also did a "tone Wheel" SPG in 1939 for 441 lines.

see http://www.tvcameramuseum.org/rca/1939fcam/1939fp4.html

Re the air friction problem the Baird scanner, film I think, ran in a reduced pressure enclosure. Alas I don't have a picture of this to hand.