Advances in electricity and a new demand for high quality boilers led to the discovery first of the Bessemer furnace and then later of the Siemens furnace and the electric furnace. With these new smelting methods, Civilization could control the carbon content of steel, the key to strength and flexibility, and produce steel in quantity for the first time. Combined with latest advances in engineering, architects could design constructions on an unprecedented scale. The Eiffel Tower is probably the most famous early building based on steel, as it was easily the tallest building of its times.