Bainbridge's mass spectrometer, shown in Figure, separates ions having the same velocity. The ions, after entering through slits, S1 and S, pass through a velocity selector composed of an electric field produced by the charged plates P and P', and a magnetic field B perpendicular to the electric field and the ion path. The ions that then pass un deviated through the crossed E and B fields and enter into a region where a second magnetic field B' exists, where they are made to follow circular paths. A photographic plate (or a modern detector) registers their arrival. Show that, for the ions, q/m = E/r BB', where r is the radius of the circular orbit.

Plate PIPT B'