The parts are now ready to assemble. First see that the metal angle contact-point is in place with the long brass strip resting on it, and that this in turn is properly fastened to the block on the side opposite the contact-point, as shown at H in Fig. 7. There should be a small hole through the middle of the brass strip directly in line with the middle of the hole in the gas-pipe. Place this back-board down on the table so that it will lie in a position as indicated in the sectional plan of Fig. 7. The gas-pipe is then to be screwed onto the plate. Over this the spool with its layers of wire is to be slipped and made fast, and the cylinder of iron is then placed in position. Over this the disk F is laid, so that the brass rod extends down through the pipe and brass strip; then the nut is screwed on to hold it in place. Next comes the front block, with its horn or bell, and the entire mass is locked together by means of the four bolts at the corners.
The wiring is simple. One inlet being through block I, the current passes through strip J to contact-point K; then through the coil and out at wire L. The inlet and outlet wires are connected to a battery and to a push-button or switch, so that the horn can be operated. The proper adjustment of this horn depends on the nuts at the foot of the brass rod. They must be screwed on tight enough to draw the strip J so that it rests on the contact-point K.
The current, passing in at I, through J, K, the coil, and out at L, magnetizes the piece of pipe and draws the iron buttons or disks attached to the tin disk. But so soon as it does so it breaks the contact between J and K, and the buttons fly back into place, having been drawn there by the rigidity of the tin disk to which they are attached. Again the current is closed and the magnet draws the iron buttons. The brass rod moves but a very slight distance up and down--enough, however, to make and break the contact between J and K. As a result of this rapid movement and the consequent snapping of the tin disk, a loud noise is emitted through the bell, which can be heard a long distance and closely resembles a long blast blown on a fish-horn.
A unique burglar-alarm trap may be made from a plate of wood, five by six inches and half an inch thick, a movable lever, and a brass strip having the ends turned out. These are arranged as shown in Fig. 8. The brass strip is fastened to the plate with screws, and the ends extend out for half an inch from the board. The lever is made from a strip of brass, and the upper part is bent out so as to clear the strip and screws that are under it. A hole is made at the lower end of the lever, through which a brass ring and the end of a spring may be fastened. The opposite end of the spring is attached to a screw, and a wire carried from it to a binding-post, A. Another wire connects the back plate with binding-post B. A string or piece of fine picture-wire is made fast to the ring and carried to any part of a room.