From the outside incandescent lamps did not change very much since the first Edison
lamps but a lot of technical fine tuning had been made on the inside. In 1911 a
filament made of wolfram had replaced the carbon filament. The precise origin of the
name wolfram is unknown but it most probably is related to the name of the Irish
chemist and mineralogist Peter Woulfe who came with the idea that the mineral
wolframite might contain a previously undiscovered element. Instead of wolfram often
the name tungsten was used, derived from the Swedish expression "tung sten"
(heavy stone), a stony mineral of wolfram. From 1913 on, the glass bulb was filled
with argon or nitrogen in order to reduce the evaporation of the filament. The
temperature of the filament could increase to about 2500 degrees Celsius, emitting
heat as well as light. Also krypton was used as a filling gas, allowing the filament to
operate at even higher temperatures thus producing an extremely bright light.