2·59 „ 0·97 × 10⁷ „ 2·72 „ 0·77 × 10⁷ „ 2·85 „ 0·63 × 10⁷ „ For the cathode rays S. Simon[130] obtained a value for _e_/_m_ of 1·86 × 10⁷ for an average speed of about 7 × 10⁹ cms. per second. In a later paper[131] with some very active radium, more satisfactory photographs were obtained, which allowed of accurate measurement. The given equation of the curve was found to agree satisfactorily with experiment. The table given below, deduced from the results given by Kaufmann, shows the agreement between the theoretical and experimental values, _u_ being the velocity of the electron and _V_ that of light. The average percentage error between the observed and calculated value is thus not much more than one per cent. It is remarkable how nearly the velocity of the electron has to approach the velocity of light before the value of _m_/_m₀_ becomes large. This is shown in the following table which gives the calculated values of _m_/_m₀_ for different velocities of the electron. Value of Observed value Percentage _u_/_V_ of _m_/_m₀ difference from theoretical values Small 1 ·732 1·34 −1·5 % ·752 1·37 −0·9 „ ·777 1·42 −0·6 „ ·801 1·47 +0·5 „ ·830 1·545 +0·5 „ ·860 1·65 0 „ ·883 1·73 +2·8 „ ·933 2·05 −7·8 „ ? ·949 2·145 −1·2 „ ·963 2·42 +0·4 „ Value of small ·1 ·5 ·9 ·99 ·999 ·9999 ·999999 _u_/_V_ Calculated 1·00 1·015 1·12 1·81 3·28 4·96 6·68 10·1 value m/m₀ Thus for velocities varying from 0 to ⅒ the velocity of light, the mass of the electron is practically constant. The increase of mass becomes appreciable at about half the velocity of light, and increases steadily as the velocity of light is approached. Theoretically the mass becomes infinite at the velocity of light, but even when the velocity of the electron only differs from that of light by one part in a million, its mass is only 10 times the value for slow speeds. The above results are therefore in agreement with the view that the mass of the electron is altogether electrical in origin and can be explained purely by electricity in motion. The value of _e_/_m₀_, for slow speeds, deduced from the results was 1·84 × 10⁷, which is in very close agreement with the value obtained by Simon for the cathode rays, viz. 1·86 × 10⁷.