What Is a Compass?

Parts of a Compass

There are many types of compasses ranging from tiny thumb compasses to complex high-tech gadgets. For most hikers and outdoors guys like us, an orienteering compass works just great and that is what we'll discuss here.

Not all compasses include each of these parts and some compasses include even more.
Baseplate	hard, flat surface on which the rest of the compass is mounted. It has a rulers on its edges for measuring distances on maps. It's edge is straight and useful for laying lines on a map
Scales	each edge of a compass may have different rulers for use with different map scales
Direction-of-Travel Arrow	marked on the base plate. You point this the way you will be traveling
Magnifier	for seeing small map features better
Index Pointer	butt end of the direction-of-travel arrow. It ends right at the edge of the dial and is where you take degree readings
Dial	ring around the housing that has degree markings engraved. You hold the dial and rotate it to rotate the entire housing
Declination Marks	use to orient the compass in an area with known declination
Orienting Arrow	marked on the floor of the housing. It rotates with the housing when the dial is turned. You use it to orient a compass to a map
Orienting Lines	series of parallel lines marked on the floor of the housing and on the base plate
Needle	magnetized piece of metal that has one end painted red to indicate North. It sits on a fine point that is nearly frictionless so it rotates freely when the compass is held fairly level and steady
Housing	main part of the compass. It is a round plastic container filled with liquid and has the compass needle inside
Bubble	a bubble of air in the housing liquid is useful for making sure you are holding the compass fairly level
Mirror	lets you see the compass face and distant objects at the same time. Useful for emergency signaling
Sight	improves aiming your compass at distant objects

How a Compass Works

There is a huge magnetic field around the earth. It is huge, but it is not very strong. The magnetized needle in a compass is aligned with this magnetic field. As the image below shows, the composition of the earth acts as a huge bar magnet sitting upside down in the middle of the planet. Since its South end is at the north pole and its North end is at the south pole, the North end of a compass needle is pulled north.
Your compass has to have a very light needle sitting on a pivot that has almost no friction. This is because the earth's magnetic field is weak and would not be able to turn the needle.