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A general equation for the rates of energy change in electronic devices at ultra-high frequencies is derived on the basis of classical electromagnetic theory, as an extension of a previous result. This provides the link between the directly observable quantities external to the electrodes and the dynamic quantities in the interelectrode space, together with radiations to remote regions. A part of these radiations is directly produced by space charges in accelerated motion. A qualitative discussion is given for the important role of the field propagation time within an electron-tube system at frequencies whose corresponding wavelengths are comparable with the tube dimensions.