Good morning;
"Light meters in the dark." Yup. E. von Hoegh has accurately described the basic way that a light sensing cell or element of a light meter works. In fact, one of the standard rated qualities for that cell is called "dark current" or how much current may flow through the light sensing cell in total darkness at a standard specified test voltage. It is inversely related to "dark resistance."
Back in the early 1960s when the then new Minolta SR-7 came out with its built-in Cadmium-Sulphide (Cd-S) light meter on the front of the camera, the owner's manual recommended storing the SR-7 in the optionally available leather case. While the manual did not specifically say why, one reason was to keep the Cd-S cell covered and dark. The SR-7 developed a reputation as "a battery eater." It seems that many American photographers were just leaving the camera sitting on a shelf or table or something, and the light meter faithfully kept metering the light of room where it was. The next year, in response to the complaints by owner's, Minolta installed a small 2 position rotary switch on the bottom of the SR-7, producing the variant I call the SR-7a. (The later SR-7b had a 3 position rotary switch with the "BC" or Battery Check position.) Then if the owner did not put the camera into the leather case, and left it on the shelf or desk, and also remembered to turn the light meter switch "OFF," nothing happened. The light meter battery lasted for a year or more.
Well, the owner's manual did suggest storing the camera in the leather case, although not explaining that you were also storing the camera light meter in the dark and saving the PX-13 or PX-625 battery. This was an incident where the ability of the average American photographer to ignore the owner's manual, and to complain about the result, did produce a modification of the camera.