The document discusses the measurement of ionizing radiation using ionization chambers. It describes how ionization chambers work by collecting ions produced when radiation interacts with air. Exposure is defined as the charge produced in air divided by the mass of air. The roentgen is the special unit of exposure. Types of ionization chambers discussed include thimble chambers, parallel plate chambers, and extrapolation chambers. Factors that affect ion collection such as voltage, recombination, temperature, and pressure are also summarized.