Select a good, clean, and flat piece of tin and cut a disk two inches and a quarter in diameter, and through the middle make a small hole. Cut two pieces of iron about the size and thickness of a cent, and bore a small hole through the centre of each. Obtain a piece of stout brass wire, or thin rod, and file one end of it as shown at G in Fig. 7, so that the small end will fit in the holes made in the iron buttons. Place one button on either side of the tin disk, and pass the wire through; then clamp it in a vise and rivet the top of the rod so that you will have a disk with a button at each side of the centre and all attached firmly to a brass rod, as shown at F in Fig. 7. The total length of this rod should be two inches and a half, and the lower end is to be threaded and provided with two small brass nuts. A piece of spring-brass three-eighths or half an inch wide is made fast to a small block at the back of the horn, as shown at H in Fig. 7, and at its opposite end a contact-piece of metal, bent at an angle, is screwed fast. Around the back of the back block a wooden frame is attached to protect the rear mechanism of the horn. 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.