the disclosure relates to a film feed speed control system in a dental radiographic apparatus for photographing the entire jaws , which apparatus has an x - ray generator and an x - ray film cassette holder mounted respectively at one end of a rotary arm of the apparatus and at the other end thereof . the system is designed to enable change in the magnification factor of an x - ray photograph by electrically and automatically increasing or decreasing the x - ray film speed independent of the x - ray generator .