Fisher M-Scope C-18 Geiger Counter (ca. 1950)
This instrument was manufactured by Fisher Research Laboratory Inc. of Palo Alto California. Since a label on the battery states that the instrument should be put into service by 1951, I would estimate that this survey meter was manufactured sometime around 1950, or a little earlier.
It is an inexpensive instrument aimed at the amateur prospector, someone who wanted to become rich but didn't want to spent a lot of money to do so.
There is no meter to display the data. The radiation levels were indicated via the headphones. The GM tube is located inside the case at the bottom. As can be seen in the photo, there is a thin plasticwindow over the tube to permit some sensitivity to beta particles.
To operate, the knob/switch (projecting from the right end of the case in the photo) is pulled out to the "On" position and a tiny button on the case (not shown) is pushed multiple times (e.g., 10 to 12 times) to "fire up" the electronics.
Detector: Thin-walled GM tube
Batteries: One 1 1/2 volt flashlight battery and one 45 volt B battery
Size: ca. 1 3/4” x 7 1/4” x 4”
Weight: ca. 2 lbs
"M-Scope" seems to have been a generic name used by Fisher to describe their line of radiation detectors. This particular model is identified on the instruction sheet as the C-18.
Fisher. Instruction sheet. No date.