Thompson's cathod ray device

Thompson's cathod ray device.

To prepare for magnetism passages on the MCAT, two types of apparatus are very helpful 'models' for what the MCAT writers will likely put before you. These are J.J. Thompson's vacuum tube apparatus and the mass spectrometer.

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.
Schematics of a typical mass spectrometer

Schematics of a typical mass spectrometer.