Fig. 968.—Tree insulator. This type of insulator is especially useful in connection with temporary or repair work, or where the wires pass through trees having numerous branches. The illustration shows a Cutler tree insulator lashed to the trunk of a tree. It is made of a single piece of glass, and is provided with a slot which the wire cannot leave accidentally. The back of the device is concave and provided with ribs which prevent sliding. It can be readily slipped over wires already in place, is available for electric light circuit, and will take wires up to ½ inch, in diameter.
Figs. 969 and 970.—Overhead cable construction. In some cases, particularly on short lines exposed to inductive disturbances from power and other electrical circuits, it is usual to string the cables on poles such as usually carry the bare conducting wires. It is not necessary, however, to insulate the cable in any way; consequently it is merely hung to a supporting wire rope or cable, called the "messenger wire," being attached either with some form of hanger, such as is shown in figs. 969 and 970, or by loops of tarred marline. The marline is sometimes wound over the cable and messenger wire from a bobbin, but frequently it is merely wound on by hand. Cables used in such overhead construction consist of bundles of wires, the pairs twisted together. The size most often used is No. 19, B. & S., which is about .03589 inch in diameter, weighs 20.7 pounds, and has a specific resistance of about 8 ohms to the mile.
Glass insulators are generally used on low tension lines, and porcelain insulators on high tension lines, the latter type being usually stronger and less brittle. Porcelain is more expensive than glass, and its opacity prevents the detection of internal defects which would be readily observed through glass.
Fig. 971.—Clark's "antihum;" a device designed to prevent the humming of telegraph wires.