A narrow, well‐collimated beam of soft x‐rays has been used for the purpose of measuring distortion in cloud chambers. The path of the x‐ray beam in the cloud chamber is made visible by the ionization of the short‐range photoelectrons. The main advantage of this method is that it can be used in the presence of a magnetic field. A limitation of the method is the finite width of the track. In argon it was not difficult to get a track 0.06 cm wide, allowing measurement to 0.004 cm. The appearance of the beam in both a diffusion and an expansion chamber is described. In addition to its use in measuring distortion, its applications to measuring rates of droplet growth and length of cloud chamber sensitive times are discussed.