A double pole-switch is shown in Fig. 12, and in general construction it is similar to the single pole-switch described above. The binding posts and bars are cut and bent from the patterns A and B in Fig. 11; but in this case the long, slim ends of the bars are omitted. A short turn is made at the handle end of each bar and a hard-wood block is placed between the bar-ends and held in position with screws driven through holes made in the bars and into the ends of the block. A handle is made fast to the middle of the block with a long and slim wood-screw; or a steel-wire nail may be passed through the handle and block, a burr slipped over the end opposite the head, and the small end riveted fast. When the binding-posts (to which the ends of the bars are attached) are screwed onto the base, be sure and see that the bars are parallel and the same distance apart at both ends. In like manner, when the cleat binding-posts are made fast, see that they are directly in line with the bars, so that the yoke will drop into the spaces between the ears without having to be pulled to one side or the other. This is a very useful switch for strong currents, and may be placed close to a dynamo, so that the current in both wires may be cut out at once. Table-jack Switches A table-jack switch is a most convenient piece of apparatus where several lines of bells, alarms, or telephone circuits are to be switched on and off. The single table-jack switch, shown in Fig. 13, is made of a hard-wood block three-quarters of an inch thick, five inches wide, and seven inches long. It is to be smoothed and varnished, or given several coats of shellac. At the four corners small holes are made to receive slim screws, and at one end of the block five short metal plates are screwed fast, with the heads of the screws countersunk, so that they will be flush with the top of the plates. These small plates should be half an inch wide and one inch long, and may be of brass, copper, or tin. But if they are of tin the plates are made of a longer strip tacked to the board and then bent over, as shown at A in Fig. 14. They will therefore form short springs, the upper parts of which will rest against the long spring-arms. From spring brass or copper five arms are to be cut and shaped, as shown in Fig. 13. Holes are made at one end of each, and others again two inches from these, through which to pass screws. Screw-eyes are passed through copper washers and the end holes in the strips, and then screwed into the wood plate. These will act as binding-posts, while the second line of screws will hold the plates down to the base. The arms should be bent, so that when the screws are driven down the lower edge will press on the small plates under them.