Physics Question #60

Sara, a female from the Internet asks on August 31, 1999,

What happens to a compass when it is held exactly on magnetic north? Does it just spin?

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The answer

James Livingston answered on August 31, 1999

In an ordinary compass, the needle can only rotate in a horizontal plane, and therefore can only respond to the horizontal part of the earth's magnetic field. When you are exactly at the magnetic north pole (which is now among the islands in the far north of Canada), the earth's magnetic field points straight down into the ground. So there is NO horizontal part of the earth's field, and the compass needle will feel no magnetic force at all. It can point in any direction you set it. There are also compass-type devices in which the needle can rotate in a vertical plane. These are called "dip needles" and can be bought from scientific supply catalogs. These respond to the vertical part of the earth's field, and would point straight down at the magnetic north pole. At other places on the earth, the needle would point down at an angle that depended on the latitude, and would lie horizontal at the magnetic equator. There are also very fancy compass-like devices in which the needle can rotate about any axis and show both the horizontal and vertical parts of the earth's field, but those are difficult to make and are very expensive.

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