Make your own TUNA-PHONES! ©

Presented at STAO '99 with a host of other physics demos :-)

John Childs - Grenville Christian College - Brockville, ON

jchilds@grenvillecc.ca -- www.grenvillecc.ca/faculty/jchilds

You can make a demo speaker out of any can, a ring-magnet, and a coil of light copper wire. The magnets are 47 cents at Radio Shack. Wind about 20 turns of "motor coil" type wire around a c-cell size battery (they just happen to be about the right size) and connect to a mini phone plug. Simply hold the magnet and coil against the bottom of a can, plug the mini plug into a source, and viola! you have a speaker. The variable current in the coil generates a variable magnetic field, which interacts with the ring-magnet field to produce vibrations that produce sound, just like a regular speaker. Actually, anything works, not just a metal can. Try plastic containers or even the table top. A fun extension to this demo is to make a pair of headphones out of two tuna fish cans. Here's how to do it...

1. Cut some plastic into strips 5 cm wide and 42 cm long. Cheap flexible vinyl 3 ring binders are a good source of plastic. To get a strip 42 cm long, you need to carefully remove the plastic from the rivets that hold the metal rings in place, so you have the entire width of the notebook. Or you can cut the strips from the height of the binder and staple two shorter pieces together. A heavy paper will also work, such as poster board. The main function of the plastic is cosmetic.






2. Remove the hook from a wire coat hanger by bending each wire repeatedly with a pair of pliers until it breaks. (Be careful as the wire gets quite hot!) Bend and form the coat hanger so it fits on top of the plastic strip. Wire coat hangers come in different thicknesses of wire. You want the thinnest wire. The thicker wire coat hangers are much too hard to work with and adjust.

3. Punch three holes in each end of the plastic strip using a hole punch. Use a nail or awl to punch four small holes where the two ends of the coat hanger meet.

4. Use two cable ties to secure the two loose ends of the coat hanger together, and at the same time fasten the coat hanger to the plastic strip.



5. Use the three holes in the end of the plastic strip as a template to mark the location for six small hoes in the tuna can.


6. Use a hammer and nail to punch the six small holes marked in step 5, in each can. These holes must be big enough for a cable tie to go through them. Also punch three more holes to secure the coils and magnets. The centre hole should be a little larger to accommodate two cable ties.



7. Fasten the plastic stipe and coat hanger to the cans using cable ties. Don't fasten them too tightly, you want them to be able to vibrate! (That's the name of the game, right?)



8. Fasten a magnet with a coil of wire under it to each can.



Do NOT hold on to the cans for this next step.


9. With both cans fastened to the wire and plastic, bend the wire in an arc that fits over your head. Do not hold on to the cans, as the small guage cable ties will SNAP! They are not strong enough to transfer the leverage for bending the wires. Use pliers if necessary, to hold onto the wires.

10. Bring the long leads from one coil over the top of the headphones to the other coil. (Twist them around the coat hanger wires to keep them secure.) Tie a knot in the black wire (the wire with the mini phone plug on one end) and secure it to the coat hanger with a cable tie.

11. Carefully scrape the lacquer insulation off about 1 cm of each lead from each coil. (Four ends.) Removing the dull orange lacquer insulation should reveal a shinny copper colour. Use a knife edge, scissors edge, or piece of sandpaper.

12. Join the two coils and the black wire into a series connection. (Parallel will also work.) Insulate the connections with some kind of tape. (Electrical tape is nice, but masking tape is fine, and in a pinch, scotch tape will also work.)



Listen to your favorite tunes!





Note: Some boom boxes don't put out much current. If you don't get much sound, or none, try another source. Some work a lot better than others. My Walkman works fine.

Materials list:
2 tuna fish cans
2 ring-magnets (Radio Shack)
2 coils of copper wire, one with leads long enough to reach over the headphones
1 mini phone plug (Radio Shack)
14 plastic cable ties - 4" size - String is OK, cable ties are convenient
1 coat hanger
1 long strip of plastic or heavy paper


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