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Electric Pendulum

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Author: Marc Rosner

Pendulums have a variety of uses in machines and technology in general.

A pendulum is a mass hanging from a fixed point by a string or other material that oscillates (swings) back and forth under the influence of gravity.

They keep the time in a cuckoo clock, they help ships navigate, and they can be used to measure acceleration. You can build a simple, inexpensive electric pendulum that will oscillate gently as long as you supply a weak electric current.

In 1851 within the dome of the Pantheon, the French physicist Jean-Bernard-Léon Foucault (1819-68) hung a 28-kg cannonball using 60 m of piano wire. On the floor, immediately below the cannonball, he sprinkled a layer of fine sand. A pointer fixed to the bottom of the ball traced in the sand, recording the movement of the pendulum. In order to keep the pendulum from wobbling, Foucault released the pendulum by burning a string that held it to one side. The pendulum made a true sweep, tracing a straight line in the sand. In a few minutes, the thin line had expanded into a pattern resembling the outline of a two-bladed propeller. The pattern grew in a clockwise direction, and at the end of an hour the line had turned 11 degrees. This could be explained only on the basis that Earth had turned on its axis beneath the pendulum, as predicted by Copernicus.

Materials

  • 1 m of thin nylon fishing line (Two-kg-test monofilament works well.)
  • 200-g mass
  • metric ruler
  • iron nut
  • 2 clamps
  • ring stand
  • 4.5 to 6 m of insulated copper wire (phone wire works well.)
  • 10-by-0.5-cm iron bolt
  • wire cutters
  • 4-volt DC power supply (You can use flashlight/lantern batteries or an AC to DC converter available at RadioShack.)

Procedure

  1. Tie one end of a nylon line to a mass. At a point 20 cm from the mass, pass the free end of the line through a nut twice so that the nut is secured in a loop when the line is taut. This will be your pendulum.
  2. Tie the pendulum to a clamp and hang it on a stand to a length of 60 cm.
  3. Make an electromagnet by winding insulated copper wire around an iron bolt. Leave 30 cm of wire free at each end, and leave 1 to 2 cm of the bolt exposed at each end. The more coils the better. You can go over the bolt with several layers—just make sure to keep winding in the same direction. Strip the insulation 1 to 2 cm from each end of the wire.
  4. Use a right-angle clamp to clamp the bolt on the stand so that it is 1 to 2 cm away from and in line with the nut when the pendulum is still.
  5. When electricity flows through a coil, it creates a magnetic field, through a process called electromagnetic induction. Electromagnets are used in electric bells, junkyards cranes, and security systems, to name just a few applications.
  6. Attach one end of the copper wire from the electromagnet to one terminal of a 4-V DC power supply. Arrange the other end of the wire so that it gently rests near the edge of the pendulum.
  7. Cut and prepare a third wire, approximately 60 cm in length. Attach one end to the free terminal of the power supply, then wrap the wire once or twice around the stand for support. Bend the wire so that the free end rests gently against the free end of the wire coming off the electromagnet. When the apparatus is constructed properly, the bare ends of the wires should touch each other gently until the mass swings into them.
  8. Give the mass a gentle push toward the electromagnet. You may have to make some adjustments for the mass to keep swinging on its own. When adjusted properly, your pendulum will go through the following cycle:
  9. Things you may want to adjust include the position of the wires, the positions of the nut and bolt, the length of the wires and number of coils, and the amount of electricity supplied to the electromagnet. (Do not exceed 6 to 8 V.)

    Electric Pendulum

    1. The electromagnet, powered by a complete circuit, pulls on the nut.
    2. This moves the pendulum toward the electromagnet.
    3. The mass hits the wire from the electromagnet, causing the circuit to disconnect, consequently ending the magnetic attraction.
    4. The pendulum swings back again, the wires touch again, and the cycle starts anew.
  10. For a real challenge, try to build a working Foucault pendulum. Then try to build a navigational pendulum that maintains its plane of orientation even when you rotate the cart upon which it rests.
Ships, planes, and submarines all employ forms of navigational pendulums. These devices help the pilot determine direction and maintain a course.
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