and thallium - green) of a mercury arc discharge to create the first metal-halide lamp can be traced to patent US1025932 in 1912 by Charles Proteus Steinmetz, the "Wizard of General Electric". The amount of mercury used has lessened over years of progress. Outer bulb Most types are fitted with an outer glass bulb to protect the inner components and prevent heat loss. The outer bulb can also be used to block some or all of the UV light generated by the mercury vapor discharge, and can be composed of specially doped "UV stop" fused silica. Ultraviolet protection is commonly employed in single ended (single base) models and double ended models that provide illumination for nearby human use. Some high-powered models, particularly the lead-gallium UV printing models and models used for some types of sports stadium lighting do not have an outer bulb. The use of a bare arc tube can allow transmission of UV or precise positioning within the optical system of a luminaire. The cover glass of the luminaire can be used to block the UV, and can also protect people or equipment if the lamp should fail by exploding. Base Some types have an Edison screw metal base, for various power ratings between 10 and 18,000 watts. Other types are double-ended, as depicted above, with R7s-24 bases composed of ceramic, along with metal connections between the interior of the arc tube and the exterior. These are made of various alloys (such as iron-cobalt-nickel) that have a thermal coefficient of expansion that matches that of the arc tube. Ballasts The electric arc in metal-halide lamps, as in all gas discharge lamps has a negative resistance property; meaning that as the current through the bulb increases, the voltage across it decreases. If the bulb is powered from a constant voltage source such as directly from the AC wiring, the current will increase until the bulb destroys itself; therefore, halide bulbs require electrical ballasts to limit the arc's current. There are two types: Inductive ballast - Many fixtures use an inductive ballast, also known as a magnetic ballast, similar to those used with fluorescent lamps. This consists of an iron-core inductor. The inductor presents an impedance to AC current. If the current through the lamp increases, the inductor reduces the voltage to keep the current limited. Electronic ballast - These are lighter and more compact. They consist of an electronic