The document summarizes the key components and functioning of a Geiger counter. It describes the three main parts: the transformer that amplifies the voltage to 400V needed by the Geiger tube, the Geiger tube which contains ionizable gas between electrodes and detects particles through ionization, and the signal stretcher which lengthens the short pulse detected to be visible or audible. It also provides background on radioactivity, isotopes, irradiation, contamination, and dose levels of radiation and their biological effects.