(Note that "elevating" the cathode voltage "above" the average heater voltage, which in well-designed equipment was supplied from a transformer with an earthed center-tapped secondary, was less detrimental to the oxide insulation between heater and cathode than "lowering" the cathode voltage "below" the heater voltage, helping to prevent pyrometallurgical electrolytic chemical reactions where the oxide touched the nickel cathode that could form conductive aluminium tungstate and which could ultimately develop into a heater-cathode short-circuit.