Exploring Magnets · Lesson 3 of 6
Magnets as Direction Finders
“Discover why freely moving magnets align north-south and how that property becomes a working compass.”
• Observe the direction in which a freely suspended magnet comes to rest. • Relate the North and South poles of a magnet to the directions they seek. • Explain how a freely suspended magnet can help distinguish a magnet from an ordinary iron bar. • Describe the structure and correct use of a magnetic compass. • Construct a simple compass by magnetising a sewing needle and allowing it to rotate freely.
A Freely Suspended Magnet Chooses a Direction
A bar magnet can reveal another important property when it is allowed to rotate freely. If it is balanced horizontally by a thread, turned gently and then left alone, it does not stop in a random direction each time. After repeated trials, it settles along nearly the same line.
Finding direction matters most when familiar landmarks or stars cannot be used. The chapter opens with sailors caught under an overcast sky, motivating the need for a device that can indicate direction even when the sky gives no help. The directional property of magnets provides that solution.
Tie a thread around the middle of a bar magnet so that it hangs horizontally. Let it become still, mark the direction of its two ends, then rotate it gently and release it. Repeat the observation several times. Compare the final direction after each trial.
The magnet repeatedly comes to rest along the north-south direction. The end that points approximately north is called the North-seeking pole, usually shortened to North pole. The opposite end is the South-seeking pole, or South pole.
The pole of a freely suspended magnet that points approximately towards the north direction.
The pole of a freely suspended magnet that points approximately towards the south direction.
Why Does the Magnet Align This Way?
The chapter explains this behaviour by treating Earth itself as a giant magnet. A freely moving magnet responds to Earth's magnetic effect, so it tends to settle in a north-south orientation. At this level, the key idea is the reliable directional behaviour, not the detailed structure of Earth's magnetism.
This property gives us a useful test. If an ordinary iron bar is suspended in the same way, it need not return to the north-south line after every turn. A true magnet, when free to rotate, shows the repeated directional alignment.
Problem
Two identical-looking metal bars are suspended one at a time. Bar A repeatedly settles along the same north-south line. Bar B stops in different directions. What does this evidence suggest?
- 1.Both bars are allowed to rotate freely, so the test conditions are similar.
- 2.Bar A shows the directional property expected of a magnet.
- 3.Bar B does not show that repeated directional behaviour.
- 4.The observations therefore support identifying Bar A as the magnet.
The Magnetic Compass
People can turn the directional property of a magnet into a practical instrument. A magnetic compass contains a small magnet shaped like a needle. The needle is balanced on a pivot so that it can rotate with very little resistance.
When the compass is placed flat and allowed to settle, its needle indicates the north-south direction. The compass box can then be rotated until the N and S markings on the dial line up with the needle. The other directions can then be read from the dial.
- Place the compass on a horizontal surface away from unnecessary disturbance.
- Wait until the needle stops moving.
- Rotate the compass box gently until the N and S marks on the dial align with the needle.
- Read north, south, east and west from the aligned dial.
Make a Simple Compass
A sewing needle made of suitable iron or steel can be turned into a small magnet by repeatedly stroking it in one direction with the same pole of a permanent magnet. The motion should be repeated from the same starting end to the same finishing end rather than rubbing back and forth.
Place a sewing needle on a wooden table. Stroke it from one end to the other with the same pole of a bar magnet. Lift the magnet, return it to the starting end, and repeat the same one-way stroke about 30–40 times. Test the needle with iron filings or steel pins. Attraction shows that the needle has become magnetised.
Pass the magnetised needle horizontally through a small cork and float the cork on water so that the needle stays above the surface. Once the cork stops turning, note the direction of the needle. Turn the cork gently and let it settle again. Repeated alignment gives the same basic behaviour as a compass.
Long before the widespread use of the modern compass, an Indian navigation device described in the chapter used a magnetised, fish-shaped iron piece floating in oil. It was known as the matsya-yantra or machchh-yantra.
When magnetising the needle, use the same pole and stroke in the same direction each time. Random back-and-forth rubbing does not reproduce the procedure being investigated.
Quiz
A freely suspended bar magnet usually comes to rest along which direction?
What makes a compass needle useful for finding direction?
An ordinary iron bar is suspended and stops in different directions after repeated turns. What does this suggest?
During needle magnetisation, which procedure matches the chapter?
After a compass needle settles, why is the compass box rotated?
Practice Problems
- Describe how you would demonstrate that a freely suspended magnet repeatedly returns to the same north-south direction.
- Explain how the suspension test can help distinguish a magnet from an ordinary iron bar.
- Write the correct sequence for using a magnetic compass to find directions at an unfamiliar place.
- Explain why the compass needle must be able to rotate freely.
- Describe how to magnetise a sewing needle using a bar magnet, including the direction of each stroke.
- Design a labelled sketch of a floating cork-and-needle compass and explain what observation would show that it is working.
Key Takeaways
• A freely suspended magnet repeatedly settles along the north-south direction. • The north-seeking end is called the North pole and the opposite end the South pole. • The chapter connects this directional behaviour with Earth acting like a giant magnet. • A magnetic compass uses a freely rotating magnetic needle to show direction. • A simple compass can be made by magnetising a needle and allowing it to rotate freely on a floating cork.