In order to determine how radioactively hot a sample is,
one must begin by calibrating the Geiger counter from a known radiation
source. Detectron provided such a source with each Geiger counter sold.
The small red disc contains a small and harmless amount of radium. Before
leaving the Detectron factory, each disc's radiation level was measured
on a precision instrument, and the reading recorded on a tag attached to
the disc. The user would then measure the calibration source and adjust
the 'counter to show the level recorded on the tag.