Wires with a single covering of silk or enamel are used when it is desirable to save space, for the covering of these two classes of magnet wires is thinner than either the cotton or double-silk-covered wire, and consequently they require less room for winding. The size of the wire is indicated by its diameter, and in the United States is measured by the Brown and Sharpe gauge, often indicated by the term, "B. & S." The preceding table shows the various sizes of wire of the Brown and Sharpe gauge, and also several of their characteristics, such as weight, resistance, etc. Insulators The covering placed over wires is not the only precaution taken to insulate them, but in the case of permanent wiring they are usually mounted on glass or porcelain supports. [Illustration: Fig. 89.—Staples and Wooden Cleat used for running Low Voltage Wires.] Wires used for batteries, bells, telephones, etc., operated by batteries and where the voltage is not over 20 volts, may be run under _insulated_ staples or wooden cleats inside of a building. If outside and exposed to the weather, they should be mounted on suitable glass or porcelain knobs. [Illustration: Fig. 90.—Porcelain Insulators to support Electric Light Wires.] Electric-light wires for inside use are commonly supported by insulators made of porcelain and known as cleats, knobs, and tubes according to the shape. Telegraph, telephone, and power lines are usually supported by glass knobs or large porcelain insulators which screw on to wooden pins. [Illustration: Fig. 91.—Glass Insulator Binding-Posts and Pin used to support Telegraph and Telephone wires.] Binding-Posts Binding-posts are the most convenient device to make quick connections between wires and other parts of electrical apparatus. Binding-posts may be either made or purchased. Those which are purchased are of course the best, and will add greatly to the appearance of any instrument upon which they are mounted. Several of the best-known types of manufactured posts are shown in Figure 92. [Illustration: Fig. 92.—Types of Binding-Posts.] Figure 93 shows different ways of making simple binding-posts and connectors from screws, washers, screw-eyes, and strips of metal. The drawings are self-explanatory and should need no comment. [Illustration: Fig. 93.—Home-made Binding-Posts.] The screws and nuts obtainable from old dry cells are very convenient to use for binding-posts and other similar purposes. Switches and Cut-Outs Switches and cut-outs are used in electrical work for turning the current on and off. If the experimenter chooses to make them himself, care should be taken, to construct them in a strong and durable fashion, for they usually are subjected to considerable use, with consequent wear and tear. [Illustration: Fig. 94.—Binding-Post removed from the Carbon of a Dry Cell.] Several very simple home-made switches are illustrated in Figure 95.