Toronto.By 1964, a new kind of exposure meter finally made it possible to record low light settings. The CdS or Cadmium Sulfide cell resistance varied with the amount of light it was exposed to. A battery with a stable voltage level allowed a consistency over time. A meter converted the light value to the correct aperture and speed for a given media (film) sensitivity (ASA or today, ISO). The CdS cell required a voltage source whereas the selenium cell generated a voltage that varied with light intensity (unfortunately the selenium cell was nearly useless under low light conditions).
The emergence of the CdS cell made cameras far more capable of auto-exposure in low light and therefore far more idiot-proof. Camera makers like Minolta in this June 12, 1964 LIFE ad (page R12, a bit after page 112) incorporated the cell to expand or maintain their segment of the retail photographic market. A simple circuit consisting of the cell, a mercury battery, an adjustable resistor (for calibration) and a meter became common place. Unfortunately pollution from expired mercury cells resulted in this popular source of stable voltage to be banned.
My thanks to George Dunbar for mentioning the LIFE ad and its highlighting of a pivotal era in photographic history. Had the Wheatstone bridge circuitry been more common, CdS meters like the Gossen Pro could have used any button cell and not been made obsolete by environmental concerns. Later on, Zener diodes and petite silver oxide cells created more expensive alternatives to physically fit spaces designed for mercury cell buttons.
In time, digital technology made all the film cameras and accessories including light meters obsolete to all but the film niche enthusiasts.