A velocity selector
When mutually perpendicular electric and magnetic fields are applied in such a way that (E+vxB)=0, the net force upon the charge vanishes. The following simulation shows 5 charged particles entering with different velocities in the region with crossed E and B fields (B perpendicular to the plane of the trajectory and E contained in that plane).


It may be observed that only one particle, that one for which the mentioned condition is fulfilled, keeps on moving without suffering any deviation. This result is used to build up a velocity selector. See experiment

By means of an experience basically similar to the previous simulation, Thomson was able to measure the ratio q/m for what he named "cathodic corpuscles". He obtained a value around 1.7 E 11 C/Kg. This happened to be the discovery of the first known subatomic particle: the electron.