When it is necessary for the high heat rating of power reactor fuel elements to be simulated in `out of pile' tests, the heating of the test element can be a major problem. The method of heating chosen for a particular rig will depend on the exact nature of the tests for which the rig is designed, and the requirements of the tests may make conventional heaters unsuitable. Very high heat fluxes can be obtained by electron bombardment and this method has been used to simulate reactor heat ratings in rig tests on a Calder Hall type element. The test element was essentially a coaxial diode; a vacuum tube representing the fuel rod was the anode, the cathode being a tungsten wire stretched down the centre. Maximum reactor heat ratings were achieved easily and it is considered that if required the power input could have been several times greater. This paper discusses the characteristics of the method and describes the important features of the design.This paper was presented at the Symposium on the Design of Experimental Equipment for use in Nuclear Energy Research, 11th-14th April, 1961, held at Atomic Energy Research Establishment, Harwell.