How Does a Compass Work?

Talk It Over

What is a compass? What is it used for? How and why does it work? You can make your own compass to answer these questions.

Get

• Modeling clay
• Drinking glass
• 2 bamboo skewers
• Rubber band
• 3 small bar magnets* or 1 long bar magnet
• Red nail polish
• Thread, about 15-cm piece
• Graph paper
• Tape
• Black, red, and green markers
• Ruler
• Map or compass (optional)

Go

1. Put a lump of modeling clay in a drinking glass. Stick a bamboo skewer upright in it.

   

2. Put another skewer across the top of the glass. Loop the rubber band around both skewers several times so they stay together, but the one resting on the drinking glass edge can still turn freely.

   

3. Put the 3 bar magnets together so they make one long magnet (or use one long bar magnet). Put a dot of red nail polish at one end so you know which end is which.
4. Tie one end of the thread around the middle of the magnet. Tie the other end to the skewer. The magnet should hang level and balanced, while still able to swing freely, like this:

   

5. Tape a sheet of graph paper to the table.
6. Set the glass on the paper, with its base centered at the left edge of the paper, like this:

   

7. With the black marker, draw around the base of the glass. Keep the glass in this circle throughout the experiment.
8. Turn the skewer so it points in any direction over the paper. With the black marker held straight down beside the skewer, make two marks on the paper: one near the glass and one near the tip of the skewer.
9. With the ruler, connect the two black dots. This line shows where you pointed the skewer.

   

10. Let the magnet swing freely. When it stops, make a red mark on the graph paper under the red-dotted end of the magnet. Make a green mark directly beneath the other end. Use the ruler and the red marker to connect the points with a straight line. Make an arrow point on the red dot end of the line, like this:

    

11. Repeat steps 8–10 several times more, pointing the skewer in a different direction each time.You should end up with a black line and a red arrow for every trial of the experiment.
12. Try to figure out where your red arrows are pointing. Use a map or compass for help if you need to.

Stay Safe

Nothing here is dangerous, but a magnet can harm your compass. Don't keep them close together for long.

Go Easy

Perform steps 1–4 of the "Go" procedure. Move the skewer to different places. Notice where the reddotted end of the magnet points with each trial.

Go Far

Modify the "Go" procedure to build a portable, build-your-own compass that will work anywhere and read direction to within a 10° (±5° at any point on the 360° in a circle) margin of accuracy.

Show Your Results

For "Go Easy," display your apparatus and present a poster that shows how and why it works as a compass. For "Go," display your graph paper records, showing the orientation of the skewer and the direction the magnet pointed in each of your trials. For "Go Far," display your homemade compass and present experimental evidence to demonstrate that you achieved the 10° accuracy required.

Tips and Tricks

• If your magnet is taking a long time to settle, it's okay to slow it with your hand. Just let go before you determine where it is pointing.
• If your magnet swings back and forth slightly, draw the line in the middle that best represents its direction.
• Keep your setup at a distance from iron and steel objects. They can affect your results

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