GM is Geiger-Muller. It is an atomic radiation meter with tubes, powered by batteries. The indicating instrument circuit was found on a Radio Craft from August 1948. Note the use of the vibrator to generate the voltage for the Geiger tube.

GM is Geiger-Muller. It is an atomic radiation meter with tubes, powered by batteries. The indicating instrument circuit was found on a Radio Craft from August 1948. Note the use of the vibrator to generate the voltage for the Geiger tube.