1.1 The main purpose of this Recommendation is to define means by which interface impedance may be measured, rather than to describe precise procedures for pre-processing, stabilizing and operating the tubes or valves to be measured, although the tube or valve operating conditions, under which the measurement of interface impedance may be made, are known to have a substantial effect on the results obtained. 1.2 It is also an objective of this Recommendation to describe means for evaluating the performance of interface impedance measuring equipment with respect to: 1.2.1 Accuracy as a function of the transconductance of the tube or valve being measured. 1.2.2 Accuracy as a function of the resistance of the interface. 1.2.3 Accuracy as a function of the time constant of the equivalent interface network. 1.3 The measuring methods and procedures described in this Recommendation are designed to provide information on the following cathode-interface impedance characteristics of tubes or valves: 1.3.1 The presence of cathode-interface impedance. 1.3.2 The equivalent resistance of cathode-interface impedance. 1.3.3 The equivalent capacitance of cathode-interface impedance. Clause 7 outlines precautions to be taken and results to be expected under certain conditions of operation of the tubes or valves being measured for interface impedance characteristics