This interesting unit was sent to me from Kieran Connelly in the USA. He had picked it up in a car boot sale for just a few dollars. Vendor information suggested that it had been designed to measure fabric thickness in a manufacturing concern. This didn't feel right to me. There was also some indication that it was a prototype for a production model and that it may have once been valuable in that respect. Again that doesn't feel right though, as evident in the pictures below, the legend PR1 has been stamped on the inside. I think that may have been wishful thinking on the part of the fabricator. I doubt there was ever a PR2.
The 594 Photronic cell has been mounted in a roughly cut aperture in the fibreboard top plate. With its mounting flange, the type 301 Microammeter is more successfully seated below. This basic arrangement emulates the ubiquitous model E703 luminance meter made in great numbers by Weston. The rheostat is not stepped but continuous and has no points of reference marked on the front panel. It could only therefor work in one way - to provide a dual scale function for the meter when turned full to either of it's poles. With the meter scaled up to 200 microamps, the unit would have needed to be calibrated and a look-up table made to return light units of some description.
All of these observations lead me to suspect that this unit was put together in America during the Second World War. No new meters would have been released by Weston to the domestic market since the company would have devoted all its production capacity to the war effort. With the need in another hard-pressed company to measure light levels (probably in the workplace), someone in the engineering shop would have constructed this from of-the-shelf components. That would be why it incorporates a general microammeter and not one marked up in light units. I am grateful to Kieran for this unit and the pictures he provided. All of those on this page are by him.
(left) the prototype designation PR1 is clearly visible on the rear of the fibreboard faceplate. (right) The WI designation (for Weston Instrument) is clearly etched onto the surface of the 594 Photronic cell. It is set in the characteristic shape of one of Weston's classic power ammeters. I have not found this on any other 594 Photronic cell assemblies.
Detail of the components mounted on the fibreboard front plate: 594 Photronic Cell, Rheostat mounted in series and the model 301 Microammeter. The assembly was mounted in a strong wooden casing.