Ionisation chamber made by Pierre Curie, c 1895-1900.
© Science Museum / Science & Society Picture Library
This device was developed by Curie (1859-1906) for radioactivity investigations. It includes a positive and negative plate connected by an electrometer, and the air within the chamber is ionised by radiation. The breakdown of air molecules into positive and negative ion pairs allows them to act as carriers of electric current. Negative ions migrate to the positive plate and positive ions to the negative plate, causing an electric current to flow. The level of radioactivity determines the current's strength.