Current Detector (Galvanometer)

Objective

The objective of this experiment is to demonstrate the relationship between electricity and magnetism. In this experiment we will also show how to create and detect magnetic fields.

Materials Required

about 8 feet of insulated wire (18-24 gauge)
metal sewing needle
3 feet of thread
permanent magnet
6 volt battery
a small strip of paper
scissors

How To Build It

Magnetize the needle by rubbing one side of the permanent magnet against it about 30 times in the same direction. Cut the paper into a small arrow. Stick the needle lengthwise into the paper arrow. Now put some thread through the top edge as shown. Using the wire, make a loop about three inches in diameter. Continue to wrap the wire in the same pattern as the original loop until you have a coil of about five loops. You should keep some wire free on both ends. Use the thread to tie the wires together. Now tie the thread attached to the paper to the top of the loop. The paper with the needle should hang freely inside the middle of the wire loop.

Experiment

Attach one end of the wire to the positive terminal of the battery, and the other wire end to the negative terminal. Notice what happens to the paper arrow. It should turn and point directly out from the middle of the loop. Now turn the loop. Notice that the arrow follows the loop and continues to point directly out of the loop.

magnetized needle in paper arrow current loop detector
[Source: Munson]
actual current detector


Troubleshooting

If your arrow does not seem to be affected when you attach the loop ends to the battery, try the following:

  1. Make sure the wires are snugly connected to the battery.
  2. Make sure that the needle is sufficiently magnetized. You might want to pull it out and stroke the magnet across it several more times. Make sure that you stroke only one pole of the magnet in only one direction across the needle. Do not simply rub the magnet back and forth across the needle.
  3. Try increasing the number of loops of wire.
  4. Replace the battery. Extended continual use of a battery can drain its power due to the low resistance of the wire.
Questions for Thought
  1. Why did the arrow turn out from the loop?
    ANSWER: Current through a wire produces a magnetic field around the wire. When current is run through a loop, it produces a magnetic field inside of the loop. Since the needle is magnetized, it tries to align itself with the magnetic field produced by the current in the loop. This is the same way a compass works. The needle inside of a compass is magnetized. That needle tries to align itself with the magnetic field produced by the earth.
  2. What would happen if we change the direction of the current?
    ANSWER: The needle would point in the opposite direction. This is because the direction of the magnetic field is dependent on the direction of the current. The direction can be "predicted" using the right-hand-rule. That is, by wrapping your right hand around the wire with the thumb in the direction of the current flow(from the positive to the negative battery terminal), the magnetic field is curled in the direction of the remaining four fingers of the right hand.
Additional Information

Several books explain this same experiment. They usually use a working compass, and simply wrap the wire into loops around the compass. The only difference is that here, we make our own "compass." This is because a needle is much easier to find and much less expensive than a compass.


ASU Department of Electrical Engineering
Last Updated: June 21, 2002
Original Page Development by: Matt Dayley and Keith Holbert
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