Electric & Musical Industries Ltd v Lissen Ltd Claim 1 of a patent was as follows: “In an amplifier circuit including an electric discharge tube wherein alternating current energy is relayed through the control action of a grid electrode upon an electron current in the vacuous space of the tube, the method of adjusting the transmission through said amplifier which comprises the steps of controlling by different amounts the portions of said electron stream flowing through different regions of said vacuous space by a special design of the tube electrodes, impressing upon said control grid a steady polarising voltage and adjusting said polarising voltage to a value substantially different from that at which the total electron current falls to zero.