This Recommendation is based on current practice of the measurement of cathode heating time and heater warm-up time. It should not be regarded as a Recommendation in the sense of a standard, because a more detailed description of the measuring methods is needed if measuring results on the basis of these principles have to be comparable within definite tolerances. Modifications of detail may be made to the circuits described provided they do not change either the principle of measurement or the degree of refinement of the circuit. Any change made should maintain the required correlation of the measuring results.