A method is described by which the radiation from a betatron can be controlled by external apparatus; such as a cloud chamber, whether the betatron magnet is energized continuously from alternating current, or is ``pulsed'' by connecting the previously‐charged capacitor bank to the magnetizing coils. Upon receiving an initiating signal from the cloud chamber, a relay and thyratron circuit actuates pulsing of the magnet if desired, and permits injection of electrons into the betatron vacuum tube only during a single cycle after the initiating signal. A series of three thyratrons delivers to output terminals a sequence of synchronizing signals at one‐half cycle before injection, injection time, and the time of x‐ray production; this permits various functions of the auxiliary apparatus to be synchronized with the betatron. By this method, a cloud chamber can be expanded shortly before electron injection into the betatron, so as to be fully sensitive at the time radiation is produced; the tracks can be illuminated and photographed within a short, adjustable interval after their formation, thereby reducing the effects of ion diffusion, distortions from turbulence, and the number of cosmic‐ray or contamination tracks.