Mass spectrograph
= Mass spectrograph
A <mass spectrometry> instrument with a photographic readout. In the 1919 apparatus described in <Isotopes by Francis William Aston/Chapter V: The Mass-Spectrograph>, a <gas> discharge produced positive <ions>. Two narrow slits formed a ribbon beam in a <vacuum>; plates applied an <electric field> to spread it, an aperture selected part of it, and a <magnetic field> bent it oppositely. Focusing reduced the spread caused by differing <ion> <speeds>, concentrating a given <mass-to-charge ratio> into a line on a photographic plate.
The observed quantity was line displacement from a fiducial spot. Known reference lines calibrated displacement against <mass-to-charge ratio>; repeated exposures with changed <magnetic field> tested the calibration. Evacuation limited collisions. Comparing line shapes with narrow and wide apertures helped distinguish geometric broadening from photographic spreading. Finite slit widths, plate response, and overlapping molecular or multiply charged <ions> limited interpretation. See <Isotopes by Francis William Aston/Chapter V: The Mass-Spectrograph> (§§33–40: apparatus and photographic procedure; §§43 and 45–46: calibration, aperture comparisons, and accuracy).
In <Mass spectra and isotopes by Francis William Aston> (Nobel lecture, 12 December 1922), the reported <neon> lines at 20 and 22 have companions at 10 and 11 from doubly charged <ions>. That paired pattern and comparison with reference lines support different <atomic masses> within one <chemical element>, rather than an <atomic mass> concentrated at the bulk average. The lecture also describes an earlier failed separation by distillation and partial separation by <diffusion>, showing why the focused <mass spectrum> supplied stronger evidence. These numbers used a historical <oxygen>-based reference scale. The lecture's whole-number rule was an empirical approximation at its stated accuracy, not the modern exact definition $A=Z+N$.<Mass spectra and isotopes by Francis William Aston>[pp. 9–13: separation attempts, focusing, reference lines, charge orders, and neon; pp. 15–17: whole-number rule and historical nuclear interpretation]{ref}