The concept of effective mass as opposed to the rest mass of the electron was reflected in numerous experiments to measure e/m with varying accelerating potentials for the electrons. The intensity of illumination of a fluorescent screen bombarded by electrons will be proportional to the number of electrons striking the screen per unit area of screen. There are two very important intrinsic features associated with electrons, namely, the fact that they are, ideally, negatively charged particles possessing a finite mass; and that an electron, or a beam of electrons, possesses a wave nature akin to that normally associated with light, x-rays, or related electromagnetic radiations. The fact that electrification of a rarefied gas produces "cathode rays" whose trajectory is made visible by luminescence and fluorescence, and which are deviated by magnetic or electric fields, was already known as early as 1869.