A spring lightning-arrester is shown in Fig. 25; it is simply a modified form of that shown and described in Fig. 19. The base-block is five by one-and-a-quarter by five-eighths of an inch, and is properly protected by a sheet of mica or asbestos. The two metal plates are cut for the binding-posts and screwed in place with screws, burrs, and screw-eyes. From spring-brass wire bend a hook and slip one end of it under the screw-head at the left side of the block. From a longer piece of wire make two or three turns around a piece of wood half an inch in diameter; then form a hook at one end and a turn at the other, so that it can be made fast under the screw-head of the binding-post. When at rest, the spring-hook should stand in an upright position, but when sprung and tied it occupies the position shown in the drawing. The spring-hook is to be bent down so that the two hooks are brought within an inch of each other. They are held in this position with a piece of lead fuse-wire. This last is given a turn about the hooks and one or two turns about itself, close to each hook, to prevent the spring from tearing itself away. When the wire is fused by a current the spring-hook flies up and away from possible contact with the short hook attached to the opposite binding-post. This is the construction for a single-pole-spring lightning-arrester; a double one is made in a similar manner, and the parts mounted on a wider block, as shown in Fig. 20. For doubtful currents, where there is no means of knowing how strong they are, a combined fuse and single-pole switch is shown in Fig. 26. This is nothing more than a combination of the apparatus shown in Fig. 21, and the single-pole switch (Fig. 10). The base block is seven inches long and two inches wide. Or it may be made half an inch wider if it is to be bevelled at the top, as shown in the drawing. It should be three-quarters of an inch thick and provided with two countersunk holes for screws that will hold it in place on a ledge or against a casement. The little angles to hold the copper-ended mica fuse-plate are described for the apparatus pictured in Fig. 21. If it is desired that one of the ends should be provided with burrs and a screw-eye, the little plate of brass should be an inch long and an inch wide, with a half-inch-square piece snipped from one corner, as shown at A in Fig. 26. It is provided with two holes, and then bent on the dotted line, so that the part with the holes will lie on the block and the ear will stand in a vertical position. A reverse-plate made on this pattern will act as one side of the switch-bar clutch at the opposite end of the block. For the metal clutch and keeper at the middle of the block the metal plate (before it is bent) will appear as shown at B in Fig. 26. The long plate with two holes lies on the base, while the first ear (or the one without the hole) forms part of the clutch for the fuse end, the ear with the hole acting as one side of the bar-lever strap.