A d.c. stream of cathode rays of constant current and speed is sent through a pair of grids between which is an oscillating electric field, parallel to the stream and of such strength as to change the speeds of the cathode rays by appreciable but not too large fractions of their initial speed. After passing these grids the electrons with increased speeds begin to overtake those with decreased speeds ahead of them. This motion groups the electrons into bunches separated by relatively empty spaces. At any point beyond the grids, therefore, the cathode-ray current can be resolved into the original d.c. plus a nonsinusoidal a.c. A considerable fraction of its power can then be converted into power of high frequency oscillations by running the stream through a second pair of grids between which is an a.c. electric field such as to take energy away from the electrons in the bunches. These two a.c. fields are best obtained by making the grids form parts of the surfaces of resonators of the type described in this Journal by Hansen.