Thomson studied the behavior of positive rays under the influence of electric and magnetic fields simultaneously. The basis design of the experimental setup he used. A beam of positive rays was subjected simultaneously to a magnetic and electric filed. The electric field (E) deflected the beam in one direction and the magnetic field (B) in a direction perpendicular to it. Due to the combined effect of these two fields, the beam travels along a parabolic curve,(curved path). The shape of the curve and the extent of the deflection of the positive rays depend upon the e/make ratio of the constituent ions in the beam.

This principle has been used in the development of a technique known as mass spectrometry. Mass spectrometry is used for identifying atomic and molecular species present in the vaporized sample of any substance. The instrument based only this principle is known as mass spectrometer.