The cathode rays are generated at the cathode _C_, and a narrow pencil of rays is obtained by passing the rays through a perforated disc _AB_. The rays then passed midway between two parallel insulated plates _D_ and _E_, _d_ centimetres apart, and maintained at a constant difference of potential _V_. The point of incidence of the pencil of rays was marked by a luminous patch produced on a phosphorescent screen placed at _PP´_. The particle carrying a negative charge _e_ in passing between the charged plates, is acted on by a force _Xe_ directed towards the positive plate, where _X_, the strength of the electric field, is given by _V_ ---- . _d_ [Illustration: Fig. 10.] The application of the electric field thus causes the luminous patch to move in the direction of the positive plate. If now a uniform magnetic field is applied at the plates _D_ and _E_, perpendicular to the pencil of rays, and parallel to the plane of the plates, and in such a direction that the electric and magnetic forces are opposed to one another, the patch of light can be brought back to its undisturbed position by adjusting the strength of the magnetic field. If _H_ is the strength of the magnetic field, the force on the particle due to the magnetic field is _Heu_, and when a balance is obtained _Heu_ = _Xe_, or _X_ _u_ = --- (1). _H_ Now if the magnetic field _H_ is acting alone, the curvature ρ of the path of the rays between the plates can be deduced from the deflection of the luminous patch. But we have seen that _mu_ _H_ = ----- (2). _e_ From equations (1) and (2), the value of _u_ and _e_/_m_ for the particle can be determined. The velocity _u_ is not constant, but depends upon the potential difference between the electrodes, and this in turn depends upon the pressure and nature of the residual gas in the tube. By altering these factors, the cathode particles may be made to acquire velocities varying between about 10⁹ and 10¹⁰ cms. per second. This velocity is enormous compared with that which can be impressed ordinarily upon matter by mechanical means. On the other hand, the value of _e_/_m_ for the particles is sensibly constant for different velocities. As a result of a series of experiments the mean value _e_/_m_ = 7·7 × 10⁶ was obtained. The value of _e_/_m_ is independent of the nature or pressure of the gas in the vacuum tube and independent of the metal used as cathode. A similar value of _e_/_m_ was obtained by Lenard[90] and others.