mass spectrograph
Mass spectrograph, device used to separate electrically charged particles according to their masses; a form of the instrument known as a mass spectrometer is often used to measure the masses of isotopes of elements. J. J. Thomson and F. W. Aston showed (c.1900) that magnetic and electric fields can be used to deflect streams of charged particles traveling in a vacuum, and that the degree of bending depends on the masses and electric charges of the particles. In the mass spectrograph the particles, in the form of ions, pass through deflecting fields (produced by carefully designed magnetic pole pieces and electrodes) and are detected by photographic plates. The beam of ions first passes through a velocity selector, consisting of a combination of electric and magnetic fields that eliminates all particles except those of a given velocity. The remaining ion beam then enters an evacuated chamber where a magnetic field bends it into a semicircular path ending at the photographic plate. The