A method of measuring secondary-electron emission from filaments

A method of measuring secondary emission which may have advantages over the usual methods in certain cases is described. The source of electrons is a tungsten filament mounted axially in a cylindrical grid and anode. The filamentary target is placed parallel to the cathode. With the grid connected to the cathode there is a considerable range of positive potentials V t and V a on target and anode respectively for which no secondaries can pass through the grid; under these conditions it is shown both theoretically and practically that the target current i t is proportional to ( V t /V a ) 1/2 . This law is then used to determine the primary currents for high values of V a such that all secondaries emitted from the target reach the anode. These, together with the measured values of i t , give the secondary emission. The possible effects of high velocity secondaries are considered. Application of the method to tungsten and tantalum gives results in agreement with published data.