Model: Photrix Small-Spot Photometer
Cell type: tbd
I've had this for a little while but haven't been able to do much with it. It was given to me by William Lampman (thank you!), but took some handling damage in-transit (thanks to the US Mail). I'm going to try to fix it but haven't had the opportunity yet, so the photo here isn't showing it off at its best.
Photrix appears to be a brand from Hickok, which made electrical test instruments (I've got some Hickok gear listed on my radio website). Along with their hand-held meters, they made this darkroom model.
Unlike selenium meters which don't require an external power source, this unit runs off standard 115V AC wall current. Underneath the panel there's a fair amount of electronics. I have no schematic diagram so a lot of this is educated guesswork, but there are two tubes; one appears to be a voltage regular (that makes sense) and the other is likely an amplifier. Since this unit would most likely be used in a darkroom, you'd need an amplifier to read the relatively low levels of light that will strike the cell.
While these never sold in big numbers, there were available from various manufacturers. Weston made a Model 603 (I don't have one), and there's a Photovolt that I suspect is another Hickok (it's in-transit. Hopefully the mail handlers won't play soccer with it), among others.
I don't have the docs for it. To use it, I would put a piece of frosted glass (i.e. milk glass) under the enlarger lens to scramble the light levels and produce an average light level, then I'd read it with the meter. I think the cell is too big to use it as a spot unless you're making huge enlargements and the area you're measuring is large enough to cover the cell.