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Fun with electromagnets

Part two: electromagnetic horn

An electromagnetic horn, operating much like an automobile horn, can be constructed from an empty metal can and few other materials. Keep in mind that this project may take a LOT of tinkering and fine adjustment to get it working!

Construction

Fasten a large can (A) to a wooden board (C). The can here is shown only in outline for clarity. A large coffee can works well. It should be clean, and have only its bottom. File down any sharp edges. Fasten it by whatever means is most convenient, be it nails, screws, or nuts and bolts (it will be difficult to nail all but the largest cans in place). It is only important that the can be firmly attached. The size of the board is dictated by the size of the can and the other components, so it will help to know the sizes of parts D, F and G before cutting C. None of the dimensions are critical, and are all dependent on the size of the can used.
Board (F) is as wide as (C) and should be about 3/4 as tall as the diameter of the can. It is nailed or screwed into place at the end of (C), on the side facing the closed end of the can. A hole is drilled through (F) approximately centered with the bottom of the can. The hole is large enough so that screw (H) can be turned through it snugly. A small space, which may need to be adjusted later, remains between the screw and the metal. Wrap a layer of paper around the screw, taping it in place, and then wind a coil around it. The coil consists of about 2 yards (2m) of insulated (preferably enameled) no. 23 or 24 magnet wire (see the electromagnet page). One free end of the wire attaches through a hole punched in the front edge of the can. The metal of the can must have its paint scraped off at this point to conduct the current, and the end of the wire needs to be stripped of insulation.
Piece (D) is either made from one piece of wood, or else it is built up from blocks. It is attached to base (C), passing into the can. A wood screw (E) passes through (D). It is not completely screwed in, leaving room for adjustment. The tip of the screw should contact the inside bottom of the can. The hole for (E) should be pre-drilled to prevent splitting of (D). The inside of the can where this screw contacts it should be scraped to the bare metal (there will be a layer of varnish). Treat this bare spot with a bit of oil.

Connections

A wire connects screw (E) to a battery (B) (start with a 6 or 9 volt battery). The other terminal of battery (B) connects to a switch (H), which can be improvised with a spring-type wooden laundry pin and two scraped metal thumbtacks. The second contact of the switch is connected to the free end of the coil (G).

How it works

When the switch is depressed, a circuit is completed, creating an electromagnet in the machine screw and coil (G and H). The bottom of the can is attracted to this electromagnet, causing it to pull away from screw (E) and breaking the circuit. Once the circuit is broken, the coil is no longer an electromagnet, and the can springs back into its original place, once again contacting the wood screw (E), completing the circuit, and the process starts all over again. This all happens very quickly, and the vibrations of the can bottom produce a horn sound.

Troubleshooting

If the horn "thumps", try backing the electromagnet away a tiny bit. If it doesn't work at all, recheck all of the connections, especially the contact with screw (E). Keep in mind that although this is a simple project, it requires a lot of adjusting to make it work. You may need to experiment with different batteries, coil windings, or materials to get it just right.
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