Considering the Lorentz forces upon free electrons, a second-order force arises due to spatial variation of the electric field-strength, which is of the same order as the conventional forces due to motion in the magnetic field and charge density fluctuations. In a continuous but not necessarily homogeneous electron distribution, such as we suppose to exist in a collision-free and neutral plasma, a small-signal theory shows that the time-averaged force per unit volume upon the electrons is,/ - - \ (pe/co2w)\7 (E -E), where p is the average electron charge density, o> = 2?r x frequency, E-E is time-averaged square of electric field, in rationalized M.K.S. units.

The total force upon an isolated body of electrons is the same as that given by conventional theory1 from the field equations ; the force is therefore consistent with the concept of radiation pressure. The internal stresses, or pressure in a fluid, differ, however, from the conventional theory, and appear to enhance the possibility of containing a heated plasma within a resonant cavity by radiation pressure2.

It is hoped that it will shortly be possible to publish the details of the calculations which we have quoted.

The existence of such a force on electrons has been demonstrated in a 10-cm. slotted anode magnetron having no magnetic field and a cold cathode. Pulses of 10-cm. radio-frequency power were fed into this magnetron and set up electromagnetic fields which decreased towards the cathode. The multipactor effect produced a copious supply of electrons within the slots and limited the radio-frequency voltage there to about 5 kV.

Pulses of electrons arrived at the cathode with energies up to 250 volts, and current of up to 0-1 amp. per cm. length of cathode could be drawn with anode and cathode at the same potential. The anode and cathode diameters were 1 -25 and 0-59 in. respectively. A small increase in cathode temperature was observed with a thermocouple, which proves that the current was due to negative particles bombarding the cathode.

Acknowledgment is made to the Admiralty for permission to publish this communication.