English: Visualisation of ion chamber operation. An incident radiation particle leaves a trail of ion pairs in its path as it travels through the gas and collides with gas atoms. Each ion pair consists of a free electron, which is attracted to the anode, and a positive ion, which is attracted to the cathode by the influence of the electric field. This movement is known as "ion drift", and ensures that the electrons and positive ions do not recombine, and that an ionisation current is generated, proportional to the number of ion pairs created.