In D a simple arrangement of a stove-bolt and two nuts is shown. The under bolt is screwed down tightly against the wood, and under the head a wire is made fast, so that another wire may be caught under the upper nut. If a small thumb-nut can be had in place of the plain nut, it will be easier to bind the upper wire. In Fig. 17 E a thin strip of metal may be folded over, and at the loose ends a hole should be punched through which a screw-eye will pass. The metal is held to a wood base with a screw, under the head of which a wire is caught. The second wire end is slipped between the metal plates, and a turn of the screw-eye will bind and hold it securely. [Illustration: FIG. 17] [Illustration: FIG. 18] Connectors are employed to unite the ends of wires temporarily, and are made in many forms. A simple and useful one is made from a piece of spiral spring fastened to a block of wood by two staples, as shown at Fig. 18 A. The ends of the wires are pressed down into the coils of the spring and are held with sufficient security for temporary use. Another connector is made from a block of wood, a strip of thin metal, and two screw-eyes (Fig. 18 B). The metal is bent around the ends of the block, and through holes made in the ends of the strip screw-eyes are driven into the block. When the ends of wires are slipped under the metal, a turn of the eyes will hold them fast, as shown at Fig. 18 B. A short bolt threaded at each end and provided with four nuts will also act as a connector. The inner nuts are screwed on tightly and the outer ones are loose, so that when wires are placed between them they may be tightened with the fingers, as shown at C in Fig. 18. These are a few simple forms of connectors; the ingenious boy can devise many others to suit his needs and ideas. Lightning-arresters and Fuse-blocks All lines of exposed wire that run from out-doors into the house should be provided at both ends with lightning-arresters, particularly if they are telephone or telegraph lines, burglar alarms, or messenger call-boxes. In many instances where unprotected telephone lines have been the plaything of lightning, painful accidents have happened, and it is only the part of prudence to provide against them. It is better to have an arrester at both ends of a line, and as the cost is insignificant it is hardly worth considering as against its feature of safety.