Quote Originally Posted by budrichard View Post
The check function is purely electronic and verifies if the meter responds to a current. Whether the GM tube is still functional and will generate a current is anyones guess without an actual radioactive source emmitting gamma rays which is what most of these GM counters were designed to detect.
It's not a "check function" that I'm refering to, but rather a check source. The check source, is a small sealed radioactive substance stuck on the side of the detector. The nuclear physics professor I bought this from told me what the substance was, but now I forget.... actinium maybe.