Toronto. When it burst on the scene in the early 1960s, the Gossen Lunasix (Luna-Pro over here) solved the problem of low light readings. Using the recently released Cadmium Sulphide (CdS) photo cell, the meter was both accurate and sensitive. Unlike its predecessor, the Selenium Photo Cell, the CdS cell did not generate an EMF (act like a battery when illuminated). Instead the cell varied its resistance as the light varied.
Gossen made a pricy meter (About $65 when I bought one) but still used a very primitive circuit. Two mercury cells were placed in series with a meter and a resistance. A switch flipped a scale and another resistor in or out to create a two scale instrument for high and low light use. A built-in translucent dome could be moved over the CdS cell to make it an incident light meter. The meter was still a big handful compared to the dainty but sturdy Weston meters.
The mercury cells by their very design hold a constant voltage over the majority of their life cycle. Governments, concerned with mercury pollution caused by discarded items, outlawed all mercury containing products including mercury cells in the late 1990s/early 2000s. Ordinary cells would have served the beautiful Luna-Pro if Gossen had chosen to use a Wheatstone bridge circuit or even a zener diode to clamp the voltage level (such an alternative was made available after mercury cells were discontinued, but for a fee of about 2/3 the original price of the meter).
15 Years ago, I took a month vacation to wander around Alberta and BC with my youngest daughter. I used my Leica and the Luna-Pro. A couple of alkaline buttons instead of the now banned mercury cells and my daughter’s Canon AE-1, calibrated my meter (I cranked the ASA until the speed and aperture matched the AE-1). Using ISO 800 colour negative film solved any minor variations in calibration over the day (in fact, the meter was relatively consistent for the whole month).
So there you have it. A few posts cover the whole gamut of how photographers determined the light value for a scene when using dry plates or roll films. After the CdS external meters came the film cameras with built-in sensitive meters, and finally digital cameras and smart phones. Of course the technologies overlapped making many options available to the photographer as the years went by.