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FunctionThis case is part of a magnetometer, which measures the strength and direction of magnetic fields. Most likely for this case, the magnetometer would be a magnetized needle or other small magnetic rod hanging from a thread coming down the tube. The needle would point in the direction of the sum-total of all magnetic fields, and by seeing its change of direction by placing one or several magnets of known strength at definite angles around it, one could deduce the unknown magnetic fields like the earth's or surrounding environment (like a ship).