Toronto. In setting a dry plate or roll film camera for a particular scene, the toughest part of all was to determine the light value so the aperture and speed could be estimated. In 1935, Weston Electric Instrument Corporation in Newark NJ (founded in 1888) developed the famous model 650 Universal Exposure Meter.
This wonderful device used a selenium cell to determine the light value then used the traditional nomograph dials to select the film speed, aperture, and shutter speed. An innovation was to use settings either side of the main pointer (a, b) and (u, o) to aid in determining the dynamic range of the scene or compensate for the meter position.
I first learned of Weston meters as a young kid when a friend of mine gave me an old Weston iron-vane “true RMS” meter (0-150v and 0-300v) once used in the General Electric factory quality assurance department at Barrie. A few years later, I acquired this DC voltmeter (0-30v) made c1900 that once graced the Collingwood, Ontario Bell Telephone Company of Canada test panel. It is the size of a dinner plate and about 4 inches deep.