Provision is made in this lamp that the rack which supports the negative carbon may be made to gear either into the teeth of the same pinion as that of the positive carbon, or into one of about half the size. By these means the light, when once focussed in a reflector, will remain in focus as long as the carbons last, whether permanent or reversed currents are employed. Besides its twofold application, the lamp is very compact, is simple in construction, and therefore not likely to get out of order, and it is capable of being regulated with great precision. There is no spring to be wound up. The contact need not be cleaned, as the sparks are scarcely perceptible. By removing two screws in the outside casing, all the chief working parts can be easily removed and inspected. Carbons are made from the hard carbon deposited in the interior of gas retorts, also from graphite. Various sizes, both square and round in section, of from 5 to 20 mm. in diameter, are used in the electric lamp according to the intensity of the electric current. Those commonly employed are from 10 to 12 mm. in diameter. The carbons supplied with the Siemens patent lamp are coated with a thin film of copper. This enhances the cost somewhat, but it greatly improves the result, as the carbons burn longer, and do not split, when so coated. By coating them the resistance is diminished, except at the points, so that all the heat is concentrated in the electric arc, and a brighter light is the result. When two dynamo machines are coupled together (see page 248), to give a very powerful current, the sizes up to 20 mm. are required. The consumption varies a little, but the average is from 3 to 4 inches per hour. [Illustration: Fig. 171.] _Concentration of Light._--Two kinds of concentrating apparatus are supplied in combination with the automatic lamp, both of which are capable of giving a powerful parallel beam, which will reach to an enormous distance, and are well adapted for naval purposes. The one kind consists of a parabolic reflector of stout metal, its concave surface being silvered and burnished. The apparatus is mounted with a ball-and-socket joint upon a wooden stand, as shown in Fig. 171. The other kind is the Fresnel catadioptric lens or holophote, Fig. 172, which may be substituted for the reflector, and gives a more powerful beam than one given by reflection. The lens is surrounded by a metal case or lantern, in which is placed the electric lamp upon a slide for focussing. Behind the carbon points a hemispherical reflector is placed, to catch all the back rays, and reflect them back through the lamp focus. The entire lantern is capable of revolving on horizontal rollers, and swings upon pivots. Two handles are placed at the back to manipulate it. [Illustration: Fig. 172.]