THE LINE. The wire or series of wires are usually supported on poles placed along the sides of a road or railway. In cities the wires are sometimes carried over houses or in underground pipes or conduits, the conductors in the ,latter case being insulated by means of a gutta-percha or othe-r suitable covering. In pole and over-house lines, the wires are kept from each other and the current from escaping to the earth by in sulators of glass. In Europe white porcelain and brown stoneware insulators are used, and the former substance, when of good quality, well glazed and well burned, is perhaps the most per fect of all insulating materials, and does not de teriorate with age. The fewer the poles on which the wires are suspended, the better is the insula tion and the less the cost, but the liability to ac cident is greater. The number of poles used varies from 30 to 50 pqr mile, and is governed by the number of wires carried, the configuration of the road or track, and other considerations. On
road lines, the number of poles is generally larger than in the case of wires carried alongside rail ways, the greater level and straightness of the latter reducing the number of supports required, and thirty-five to forty to the mile is considered an average. The wire chiefly used for overland telegraph purposes is of iron, galvanized, and of No. 8 (1-8 inch) or No. 6 (1-6 inch) gauge, or copper of somewhat smaller diameter, the lat ter being preferred on account of its greater con ductivity. The conductivity of a wire increases in the ratio of the square of its diameter (the resistance decreasing in inverse ratio), and the advantage of using a thicker wire on the longer lines is thus seen. No. 4 wire is, for this reason, used on some of the longer lines.