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How to Make a Carbon Microphone

In the 1870s David Edwards Hughes invented the carbon microphone. Carbon microphones consist of a moveable metal plate and a non-moveable metal plate separated by carbon grains. When sound strikes the moveable plate, it vibrates the carbon, changing the resistance in the carbon. The change in resistance is read by electrical equipment to convert the sound into an electrical signal. While the carbon microphone has largely been replaced by more modern technology, it is simple enough to build for a science fair or as a fun project.

Things You'll Need

  • Aluminum foil
  • 2 plastic cups
  • Scissors
  • Glue stick
  • Activated carbon
  • Computer speakers (optional)
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Instructions

    • 1

      Cut two strips of aluminum foil with the scissors. They should be slightly narrower than the base of the plastic cup and one and a half times as tall. Trim the corners off one end of both strips.

    • 2

      Spread a thin layer of glue on the outside bottom and in a strip on the outside of the first cup. Apply one foil strip on so it covers only the bottom and runs up the outside. Use the end of the foil with trimmed corners on the bottom of the cup so that the corners do not extend on to the side. Press the foil so the glue holds it firmly against the cup. The more firmly the foil adheres to the cup, the more effective the finished microphone will be.

    • 3

      Spread glue on the inside bottom and in a strip on the inside of the second cup. Press the second foil strip inside with trimmed corners at the bottom so the foil covers only the bottom and the strip of glue. Like the first strip, the second will have to be pressed firmly so the foil makes good contact.

    • 4

      Pour granulated activated carbon into the bottom of the second cup to a depth of one millimeter, or until the aluminum foil is completely covered. There must be enough to make good contact with both strips of foil and prevent them from touching.

    • 5

      Place the first cup into the second cup. Make sure the foil on the bottom of the first cup does not come into contact with the foil on the second cup. This will cause a short circuit, bypassing the carbon and preventing the microphone from working.


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