Discovered in 1871 by P.J. Hjelm in Uppsala, Sweden. Molybdenum is a lustrous, silvery coloured metal which has an abundance of 1.5 ppm in the earth's crust. In many instances, it shows a resemblance to tungsten with which it tends to be paired in the transition series in the periodic table, but their chemistries tend to show more distinct differences than might be expected. Molybdenum has a high melting point and applications for the pure metal take advantage of this; for example, the pure material is used as resistance heating elements in furnaces, as filament supports in electric lamps, and as electrodes for mercury vapour lamps. Molybdenum is used as an alloying agent in certain grades of steel, Permalloys and Stellites (a series of alloys which contain varying proportions of Cr, Co, W and Mo, are very hard and are used in cutting tools and to protect surfaces subject to heavy wear).Rondella - Un sottile, piatto pezzo di materiale con un foro nel mezzo, utilizzato in combinazione con bulloni e dadi per distribuire il carico di un dispositivo di fissaggio filettato.