geiger
A Geiger counter provides a sensitive means of detecting radioactivity. A tube is filled with an inert gas, which will conduct electricity when radiation enters it. When a charged particle comes into the tube, it changes the electrical potential between the anode and the cathode. This change in potential in the tube produces a change in voltage in the electrical circuit and registers as a count. Geiger counters are fairly inexpensive and reliable, so they are useful in a wide range of applications. More complicated types of counters are also available, but are generally used in sophisticated experiments.