Bainbridge mass spectrometer

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A Bainbridge mass spectrometer is a device used to determine isotopic masses. A beam of positive particles is produced from the isotope under study. The beam is subject to the combined action of perpendicular electric and magnetic fields. Since the forces due to these two fields are equal and opposite, the particles with a velocity equal to v = E / B do not experience a resultant force. So, they pass freely through a slit, and are then subject to another magnetic field, transversing a semi-circular path and striking a photographic plate. The mass of the isotope is determined through subsequent calculation.

N.B.: E is electric field strength; B is magnetic field strength.