In J.J. Thompson's vacuum tube apparatus, a cathode ray is beamed into a vacuum chamber in which both an electric and magnetic field have been established. The field strengths can be adjusted to create a situation of force equilibrium between the electric and magnetic force on a particle. Using this force equilibrium, Thompson was able to determine the charge to mass ratio, q/m, of the electron.
Likewise, the behavior of charged particles moving through a magnetic field is central to mass spectroscopy. The mass spectrometer separates ions according to their mass-to-charge ratio. Charged particles are first sent through a velocity selecter, then they enter a region where a magnetic field causes the ions to move in a semicircular path and strike a photographic film.
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