Compasses

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A compass is an instrument for determining directions, comprising a freely rotating magnetised needle that indicates magnetic North. The compass, when used away from permanent magnets will point to the Geographic North Pole. The North point on the compass is a Magnetic North Pole. Using the rule of unlike Poles attract, this means that the Geographic North Pole is actually a Magnetic South Pole (and hence the Geographic South is a Magnetic North). This convention is used by all scientists and engineers. Any other definition is technically incorrect.

When a compass is placed against a Pole face of a permanent magnet, how the North point of the magnet aligns relative to the Pole face indicates the polarity of the magnet. If the North point of the compass points to the Pole face, that Pole face is a South Pole on the Magnet. If the North point of the compass points away from the Pole face, that Pole face is a North Pole on the Magnet. This still follows the like Poles repel, unlike Poles attract rules. And therefore the North Pole on the Magnet, if it were suspended and allowed to rotate to be in line with the planet Earth’s magnetic field, would point to the Geographic North Pole (it is a “North seeking” Pole). This is correct as we already know that the Geographic North is a Magnetic South Pole.

Field lines can also be ‘seen’ using compasses. Place a magnet on a piece of paper (perhaps draw the magnet profile on the paper to allow repositioning of the magnet). From the North Pole face, put a small pencil mark from the magnet pole. Put the compass to the North Pole face such that the South point of the compass points to the pencil mark. Add a new pencil mark where the North point of the compass is. Move the compass so the South point of the compass is where the new pencil mark has been made and repeat the process of marking where the North point is. Continue this process and note how the compass starts to move around the paper until it meets with the South Pole of the magnet. Then join the pencil marks together and put some arrows on the joined up line such that the arrows point towards the South Pole. The result is a magnetic field line plot. If you repeat this process but starting the compass at different locations on the magnet pole face, you will start to see additional magnetic field lines. If you then put ferromagnetic steel near the magnet and repeat the process, you will see the effects on field lines when materials with high magnetic permeabilities are present, taking the understanding of magnetism to a higher level and allowing the basics of magnetic circuits and magnetic circuit design to be understood.

We offer two compass sizes. The smaller compass (DC204) is a rotating disc and is ideal for field line experimentation. The larger compass (IS300) is of a more traditional magnetic needle design and has 5 degree markations to allow for more accurate measurements and is ideal for use when navigating using maps (to determine the direction the Earth’s North is). Either compass can be used as means for detecting the polarity of a magnet.

For note, it is possible to accidentally remagnetise a compass (so the North is pointing the wrong way). This happens if a strong magnet is accidentally swiped over the top of bottom of a compass. To correct, get a strong magnet and put it against the compass so the pointer is strongly directed to the magnet. Then move the magnet pole under the compass (along the line of the needle, trying not to move the needle) and bring it to the other side of the compass such that the same magnet pole is facing the compass. Remove the magnet and re-test the compass. The compass should now be reading North correctly (the process described has remagnetised the compass with the correct direction of magnetisation). If the compass is used to test magnet polarity and it is critical to know the correct polarity at all times, an electronic polarity checker can be used instead (we offer this product).