This equipment displays the number of atoms in a given quantity of radioactive material that are detected to have decayed in one second. The radio active source is held in a Perspex source mount on a wooden ruler base board facing a Geiger-Muller tube. The tube contains a gas which is ionised by the energetic radiations produced by the sample. This sets up a cascade of electrons, resulting in an electric pulse. These pulses are counted by the Ratemeter and displayed on an analogue dial as counts per second (CPS). The ruler base allows the fall in radiation exposure with distance to be investigated.