Physicist J.J Thomson measured the ratio of electrical charge (e) to the mass (m) of a cathode ray particle by using a specially designed cathode ray bulb. He was able to determine the value of e/m. It is most widely used in the electrodynamics of charged particles. The e/m experiment demonstrates the very important concept of F = q(v x B ) on a charged particle. This application is a conceptual simulation of the above experiment. Very useful for students and teachers alike.