Making a Practical Compass Part 1
by Jim Hansen
This is part one of a project that requires serious student focus to resolve. Here we show how easy it is to make a compass, but it is one with some bad habits that will have to be resolved. Even so, as a compass it is almost there. Our goal is to make a practical compass that could be used on a sailboat on a day trip and this is our first prototype, one that is very loosely modeled after a binnacle compass.
There are any number of web sites that describe how to make a compass. It is a seemingly trivial matter. In so many words typical instructions are "place a magnetized needle on a cork or piece of styrofoam floating in a pan of water." If the goal is to instruct elementary students in following a procedure, this is probably okay.
But if the goal is to make a working, practical compass through an exploration, the project is a total failure. The cork will gradually float against the edge of the pan, and the “magnetized” needle, created by rubbing it against a magnet, isn’t strong enough to give a positive swing toward the right direction. Any motion at all, any draft or breath of air against the cork sends the needle swinging in random directions. And the compass isn't "portable."
A binnacle compass is a large compass mounted to the deck of a ship (on the "binnacle") near the ship’s wheel. (The photo above was found on an eBay sale site.) The compass is watertight and protected from the elements of the sea. There are several variations, but all are viewed by the helmsman from a standing position, and most have a small oil lamp mounted on one side of the compass for night viewing.
The following is a “prototype” solution. The challenge is for students to make a construction of their own design that avoids some of the problems revealed by this model. You can make this project more challenging by having students design their own float or develop the magnet for it themselves. If built as described here, this compass is responsive and works very well, but it has problems which are to be worked on in a revised version.
The two artistic elements in this project are the compass rose (this didn’t appear in the west until the 1300s) and the compass case. Students should be encouraged to design their own compass rose and to be able to name at least the eight cardinal points on it. Their design will be copied onto a overhead transparency (to make it waterproof) and they will cut it out and mount on their compass.
Finding a suitable alternative to the soda bottle compass case will be difficult. The fundamental requirements for the case are these: provide a pool of water for the compass to float in, allow for viewing the compass, provide for portability, and be made with classroom-acceptable materials and tools. Some students may want to make theirs from clay or ceramic, and I would encourage this originality provided they can stay on schedule.
For this project you will need:
1 ea empty 2-liter soda bottle
1 ea disposable petri dish
1 ea overhead transparency film
1 ea small neodymium magnet
Small amount of Gorilla Glue or
other permanent waterproof glue
Start by cutting your soda bottle into two sections. Draw a line around the bottle just below where the top section stops curving, then cut the top free. Peel off as much of the soda label as possible from the remaining bottom section, then locate the raised pattern around the base, about 2.5 inches from the bottom. Draw a line about a quarter inch above this pattern, and cut the bottom from the remaining center.
Trim the edges from the top and bottom sections to be as smooth as possible. Because the diameter of the top portion of soda bottles is a few thousandths of an inch smaller than the bottom, with care it is possible to insert the top section into the bottom section. The result is somewhat of a “classic” container shape which will soon become our compass body.
The compass rose should now be drawn by the students, but since we are teachers we have greater flexibility. I found this one at http://www.nappf.com, a website of the North American Powered Parachute Federation. This image should be printed with a diameter of about 3.5 inches (any size smaller than the inside of the soda bottle) onto a piece of overhead transparency stock, then smoothly cut out. (See full resolution image in the Notes section.)
Center and glue the compass rose to the top of a plastic disposable petri dish using a single drop of glue in the center. The top of the dish is clear and has a raised lip around the edge (see photo below), and when the compass rose is glued in the center, the lip removes the natural curl in the film, forcing it to lie flat. Place a small weight to press the compass rose to hold it firmly against the top and give plenty of time for the glue to cure.
The heart of any magnetic compass is the magnet. Use any small neodymium magnet. These can be bought from almost any magnet dealer or on Ebay for something less than 10 for a dollar. Don’t be tempted to try using anything else; the stronger the magnet the more directional and responsive the compass.
However, compass sensitivity comes at a price: it makes the compass more sensitive to magnetic materials around it, and in a real-life situation, the overall accuracy of the compass could be affected. The two huge iron balls on the binnacle compass shown above are adjusted to compensate for the magnetic properties of the ship. This entire topic can be made into an interesting classroom discussion.
Mount your magnet in the exact middle of the bottom of your petri dish. Don’t worry about the direction it faces as this will be adjusted later. The magnet can temporarily be positioned using glue stick, then permanently attached after the compass has been assembled and tested.
Fill the bottom of the compass case with water, then float the magnet assembly and observe which end faces north. Remove the magnet from the water and assemble the compass rose cover over the magnet with the north indicator facing the correct direction. Float the completed assembly again and verify that the compass rose points north. Make additional adjustments as necessary.
Install the compass top by carefully fitting it inside the bottom, then inspect the work and check out the compass performance.
Problems that should be noted:
Although this compass "works," there are a number of inherent design flaws. What are its shortcomings? Every student (or student team) should be allowed time to completely examine this compass and search for the defects by themselves.
Some of the more obvious issues are listed below to get you started by suggesting topics for classroom discussion prior to students starting work on their own design. Students should record these observations into their lab notebooks and understand that part of their job will be to uncover these inadequacies and to design them out of their product in a future design. For now, we're interested in the students developing an analytical understanding of what they are making, knowing that improvements can be made...and they have permission to try right off!
1. This compass leaks if tipped. The top and bottom could be sealed after assembly, but what if it needed to be repaired? The compass could also be mounted so it can’t tip, or perhaps it could be mounted in a swing. (If so, then ask what would happen in a storm - could the helmsman actually use a swinging compass?
2. The compass rose keeps moving over to the side of the case and “sticking.” This is a problem common to this type of compass. What causes this? Could it be that the compass is trying to float on the top of a bubble formed by surface tension? How could it be stopped from falling off the bubble? Would a little detergent in the water help? Possible solutions include making a wire ring or cage that captues the petri dish and holds it in the middle of the water. Would that make the compass stick as well? Let the students figure out the best solution by experiment.
3. Or perhaps a different float scheme that would be centered by a wire under the compass center of rotation. The water can also be removed and some other sort of mechanical pivot used. These ideas would keep the compass better centered, but might introduce new problems.
4. The compass rose itself is in the water; maybe that is causing problems with surface tension and is related to problem 2. This possibility chould be investigated by placing some styrofoam floats under the compass.
5. The compass cannot be conveniently carried. Perhaps carrying straps could be added or the top cover could have a hook or ring-eye mounted on it.
6. The compass cannot be used at night. A night light of some sort is needed. Student creativity should probably be directed towards a flashlight or LED lighting system, rather than the oil lamps traditionally used.
These and several other notes are addressed in Binnacle Compass, Part 2.
Notes
The following templates are provided for instructor use.
This is a mirrored version of the same compass rose. The dull side (printed side) will face down when mounted on the magnet assembly.
