A Floating Compass

posted on 28 Jan 2013 by guy
last changed 22 May 2015

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ages: 9 to 99 yrs
budget: $1.00 to $5.00
prep time: 5 to 10 min
class time: 15 to 60 min

This lesson provides

  • plans for making a simple compass and
  • tips on magnetizing a needle

learning goals:

  • understand the mechanism behind magnetization
  • figure out which way is north

For other ideas and more in-depth explorations of magnetism, check out our curriculum summaries of lessons on Magnets and Materials and Magnets and Motors.

 

required equipment: magnet, steel needle
subjects: Geology, Physics



Video by minimalistsurvival at youtube with instructions for making a simple compass.

the principle of the compass

Any magnet that is free to rotate can act as a compass.  Two magnetic poles of opposite type attract each other, so a magnet on the surface of the Earth will try to orient itself so that its north pole is facing the Earth's magnetic pole in the arctic (which is actually a south magnetic pole). See our lesson on Earth's Magnetic Field for more details.

making a floating compass

The components of a magnetic compass are very simple. All you need are a magnetized piece of steel or iron (like a needle) and a way for it to orient freely in the earth's magnetic field. The video above by minimalistsurvival gives a particularly clear demonstration of how to do this by floating a needle on a leaf.

A few comments are in order regarding the best way to magnetize a needle; understanding the underlying mechanism of magnetization is helpful. (See our lesson on Iron and Magnets for even more details.)

Iron (and other magnetic materials like nickel and cobalt) is composed of microscopic regions called "domains" where the magnetic fields of the atoms all line up in the same direction. In unmagnetized iron, the domains all point in random directions, so that the domain fields tend to cancel each other and the net magnetic field of the iron is neglegible. However, when the iron is subjected to an external magnetic field, the domain fields tend to align in the same direction, just like a compass needle aligns in the earth's magnetic field. In this case, all the domain fields reinforce each other and the net magnetic field of the iron can be quite substantial. When the external field is removed, many of the domains will return to their random orientations, but in some materials many of the domains will stay aligned in the same direction, giving a (sometimes very strong) residual magnetic field.

For a long thin piece of steel, like a needle, the most stable configuration occurs when the magnetic field is along length of the needle. Each domain's field is strongest right at the edge of each pole, and in this configuration, the north pole of one domain butts right up against the south pole of the next domain, all the way down the entire length of the needle. In this way, the domain fields hold each other in alignment, and are most resistant to heat or jarring, which might disrupt the alignment and demagnetize the material.

As demonstrated in the video, one reliable method of magnetizing a needle is to set the needle against the side of a magnet with the tip of needle near the south pole and the eye of needle near the north pole. As it's attracted to the magnet, the tip of the needle is closest to the south pole of the magnet and itself becomes a north magnetic pole; the eye is closest to the north pole of the magnet, and becomes a south pole. Putting the needle and magnet in the refridgerator helps prevent the magnetic domains of the needle from being randomized by thermal vibrations.


Fig. 1: Magnetizing a needle by holding it near one pole of a strong magnet.

Another simple method is to hold the needle with the tip pointing into the south pole of a strong magnet for a minute or so (as in figure 1). This is where the field of the magnet is strongest, and it ensures the tip of the needle will form a north pole. Flicking the needle with your fingernail seems to help, perhaps by shaking up the domains and letting them settle into their proper orientation.

Conventional wisdom has it you can also magnetize a needle by stroking it along the length of a magnet, but this technique is unreliable. If you stroke along the entire length of the magnet, you are moving the needle from the south pole to the north, alternating the magnetic alignment forces on the domains. The effect on the needle tends to cancel with each full stroke. If you stroke the needle only near one pole of the magnet, always holding the needle in the same orientation, you can usually get the needle to magnetize, but I find it simpler and more reliable to just hold the needle next to the pole as in figure 1.

When you're done you can test how strongly your needle has been magnetized by trying to pick up a paper clip with it. If you can lift the paper clip at least partially off the table, your needle is plenty strong enough for a compass.

The only remaining step to making a compass is to mount your magnetized needle so that it is free to rotate. Float it on a leaf or a paper boat, or hang it from a string.

question to ponder

  • What direction would a compass point at the north geographic pole?

At the north geographic pole a compass would point straight down. In fact, throughout the northern hemisphere, a compass needle will try to point north and down; in the southern hemisphere it tries to point south and up. The actual declination angle at any location on earth can be looked up using a geomagnetic field calculator. To prove to yourself that the field in the northern hemisphere points slightly down, hang an unmagnetized needle on a string and mark the balance point where the needle rests horizontally. Then magnetize the needle and rehang it from the same balance point. The needle should now point its north pole somewhat towards the ground.


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