(Since the electric particles travelling in the direction of the rays are negative, and since it is customary by the expression “direction of current” to understand the direction opposite to that in which the negative electricity moves, then, in the case of the cathode rays just mentioned, the direction of the current must be opposite to that of the rays.) From measurements of the magnetic and electric deviations it is possible to find not only the velocity of the particles, but also the ratio _e_/_m_ between the charge _e_ of the particle and its mass _m_. The velocity varies with the potential at the cathode, and may be very great, 50,000 km. per second, for instance (about one-sixth the speed of light), or more. It has been found that _e_/_m_ always has the same value, regardless of the metal of the cathode and of the gas in the tube; this means that the particles are not atoms of the elements, but something quite new. It has also been found that _e_/_m_ is about two thousand times as large as the ratio between the charge and the mass of the hydrogen atom in electrolysis. If we now assume that _e_ is just the elementary quantum of electricity 4·77 × 10¹⁰, which in magnitude amounts to the charge of the hydrogen atom in electrolysis (but is negative), then _m_ must have about ¹/₂₀₀₀ the mass of the hydrogen atom. This assumption as to the size of _e_ has been justified by experiments of more direct nature. The experiments with charge and mass of electrons which have in particular been carried out by the English physicist, J. J. Thomson, give reason then to suppose these quite new and unknown particles to be free atoms of negative electricity; they have been given the name of _electrons_. Gradually more information about them has been acquired. Thus it has been possible in various ways to determine directly the charge on the electron, independently of its mass. Special mention must be made of the brilliant investigations of the American, Millikan, on the motion of very small electrified oil-drops through air under the influence of an electric force. To Millikan is due the above-mentioned value of _e_, which is accurate to one part in five hundred. Further, the mass of the electron has been more exactly calculated as about ¹/₁₈₃₅ that of the hydrogen atom. Their magnitude has also been learned; the radius of the electron is estimated as 1·5 × 10⁻¹³ cm. or 1·5 × 10⁻⁶ μμ, an order of magnitude one ten-thousandth that of the molecule or atom.