The extreme electronegativity of sulphur means it is fiercely corrosive towards many materials found in other lamps. Tungsten electrodes are attacked and destroyed very rapidly, thus it is necessary to create an electrodeless lamp. Power is coupled to the discharge using a magnetron operating in the microwave frequency range, but efficiencies of 70% at best were attainable here. Because of the large bulb size convection currents are also a serious issue, and to deliver the temperature uniformity required to attain full vapour pressure of the sulphur (5 atm.), it is necessary to slowly rotate the bulb horizontally about the axis of the quartz support rod. The argon filling acts as a starting aid.