Just behind the yoke, and directly under the armature-bar, a long screw is driven into the base-block, as shown at B in Fig. 5. It must not touch the yoke, and the head should be less than one-eighth of an inch below the bar when at rest. On this the armature-bar strikes and clicks when drawn to the magnets. The armature must not touch the magnets; otherwise the residual magnetism would hold it down. The screw must be nicely adjusted, so that a loud, clear click will result. [Illustration: FIG. 2 FIG. 3 FIG. 5 FIG. 6 FIG. 8 TELEGRAPH KEY AND SOUNDER] When the sounder is at rest the rear end lies on the brass tack in the block, and the armature is about a quarter of an inch above the top of the magnets. The armature is of soft iron, two inches and a half long, seven-eighths of an inch wide, and an eighth of an inch thick. These small scraps of metal may be procured at a blacksmith’s shop, and, for a few cents, he will bore the holes in the required places; or if you have a breast or hand drill the metal may be held in a vise and properly perforated. By connecting one wire from the key directly with one of the binding-posts of the sounder, and the other with the poles of a battery, and so on to the sounder, the apparatus is ready for use. By pressing on the key the circuit is closed, and the magnetism of the sounder-cores draws the armature down with a click. On releasing the key the bar flies back to rest, having been pulled down by the spring, and it clicks on the brass tack-head. These two instruments may be placed any distance apart, miles if necessary, so long as sufficient current is employed to work the sounder. Two sets of instruments must be made if boys in separate houses are to have a line. Each one must have a key, sounder, and cell, or several cells connected in series to form a battery, according to the current required. In the plan of the telegraph-line connections (Fig. 7, page 196) a clear idea is given for the wiring; and if the line and return wires are to be very long, it would be best to have them of No. 14 galvanized telegraph-wire, copper being too expensive, although much better. These wires must not touch each other, and when attached to a house, barn, or trees, porcelain or glass insulators should be used. If nothing better can be had, the necks of some stout glass bottles may be held with wooden pins or large nails, and the wire twisted to them, as shown in Fig. 8. When the line is not in use the switches on both keys should be closed; otherwise it would be impossible for the boy having the closed switch to call up the boy with the open one. Take great care in wiring your apparatus to study the plan, for a misconnected wire will throw the whole system out of order. [Illustration: FIG. 7]