L.R. Ingersoll Wonders of Physics Museum
UW–⁠Madison Department of Physics

Circle of Magnetism

{Updated photos coming soon}

 

This exhibit is designed to help you understand the right-hand-rule and will help you understand the connection between current and magnetism. Now imagine grabbing the vertical wire with your right hand with your thumb pointing up. In this case your fingers would represent the direction of the magnetic field curling around the vertical wire (counter clockwise) and your thumb would indicate the direction of the current that is flowing through the wire.  The large disk with the compasses can rotate 360°, which will help you visualize the magnetic fields via compass needles which should always point in the same direction. There are two buttons on either side of the large rotating disc.  The red button is for power, the black button flips the direction of the current within the wire. The system will automatically shut off after a set time. This exhibit is also known as the Oersted’s Experiment. Named after a Danish physicist in 1820, Hans Christian Oersted, that discovered a relationship between electricity and magnetism well giving a special lecture on electricity.

 

Before pushing the power button:

Observe: Notice how all the compass needles point in the same direction!  Which happens to be the magnetic field of the earth! (ie. North-South direction) Also notice that the compass needles continually pointing in the same direction as you rotate slowly the large disk.
Now Do: Turn on the power by pushing either red button that are located on the side of the rotating disk.
Observe:

 

 You’ll notice that all the needles will point in a circle and for the most part in the same direction. But which direction, clockwise or counterclockwise?  Also notice that the circular patterns of the compass needles seem to be tighter in the center and weaker at the outer edge. Why? Can you apply the right-hand-rule to determine the direction of the current?
Now Do: Now push the black button!!  What happened and why?
Observe:

 

 You should have seen all the compass needles flip their direction and point the other way. No matter how you rotate the large disc, they continually point in that new direction. Push the black button again and the compass needles revert back to the original direction. Try to apply the right-hand-rule!
What’s Happening: When you apply the power you are applying approximately 30 amps worth of current through the wire. What you cannot see is there are three loops of that wire through the stainless steel tube. So the current flows through the stainless steel 3 times, this helps increase the magnetic field that you are seeing with the compasses. We can apply the right hand rule with the direction of the compass needles. Thus line your fingers with the Red Arrows is your thumb pointing up or is your thumb pointing down? Your thumb tells you which way the current is flowing through the wire.  The needles are weaker on the outside because the magnetic field or flux is weaker the farther away you are from a current carrying wire.

 

 

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