When the carbon rods are brought into contact and then slightly separated, a spark passes between them. Particles of carbon are torn off by the spark and volatilized, and these incandescent particles form a sort of bridge which is a sufficiently good conductor for the current to pass across it from one rod to the other. When the carbons are placed horizontally, the glowing mass is carried upwards by the ascending currents of heated air, and it assumes the arch-like form from which it gets its name. If the carbons are vertical the curve is not produced, a more or less straight line being formed instead. The electric arc may be formed between any conducting substances, but for practical lighting purposes carbon is found to be most suitable. Either continuous or alternating currents may be used to form the arc. With continuous current, if the carbon rods are fully exposed to the air, they gradually consume away, and minute particles of carbon are carried across from the positive rod to the negative rod, so that the former wastes at about twice the rate of the latter. The end of the positive rod becomes hollowed out so as to resemble a little crater, and the end of the negative rod becomes more or less pointed. The fact that with continuous current the positive rod consumes away twice as fast as the negative rod, may be taken advantage of to decrease the cost of new carbons, by replacing the wasted positive rod with a new one, and using the unconsumed portion of the old positive rod as a new negative rod.[1] If alternating current is used, each rod in turn becomes the positive rod, so that no crater is formed, and both the carbons have the same shape and are consumed at the same rate. A humming noise is liable to be produced by the alternating current arc, but by careful construction of the lamp this noise is reduced to the minimum. [1] In actual practice the positive carbon is made double the thickness of the negative, so that the two consume at about the same rate. If the carbons are enclosed in a suitable globe the rate of wasting is very much less. The oxygen inside the globe becomes rapidly consumed, and although the globe is not air-tight, the heated gases produced inside it check the entrance of further supplies of fresh air as long as the lamp is kept burning. When the light is extinguished, and the lamp cools down, fresh air enters again freely. Arc lamp carbons may be either solid or cored. The solid form is made entirely of very hard carbon, while the cored form consists of a narrow tube of carbon filled up with soft graphite. Cored carbons usually burn more steadily than the solid form. In what are known as flame arc lamps the carbons are impregnated with certain metallic salts, such as calcium. These lamps give more light for the same amount of current. The arc is long and flame-like, and usually of a striking yellow colour, but it is not so steady as the ordinary arc.