Definition of Ballast
Broadly described, ballast is something which enhances stability or evenness. It was originally a nautical term but has been extended into other contexts as well, such as electronic ballast.
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Ships and Ballast
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The distribution of mass within a ship, as well as wind and waves, can cause it to heel (lean) to one side. Heavy objects---ballast---placed at the bottom of a ship give it a low center of gravity and limit its ability to heel. Closely related to this is the use of ballast to control the buoyancy of submarines and balloons.
Ballast Materials
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Anything heavy can serve as ballast. Historically stone or sand was often used. Modern ships frequently use water. They have special compartments which can be filled or emptied as needed, even while underway.
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Invasive Species
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When a ship fills its ballast tanks with water it can bring aboard plants and animals too. These are released into the wild again when the ballast is emptied. It is believed that a number of species---like the zebra mussel---have been introduced into new habitats this way. New methods of ballasting ships are being sought.
Roadbeds
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Ballast is also used to describe a layer---typically of rocks or gravel---laid down as the bed for a railroad or road. It provides a stable and level platform for the rails or road surface.
Electrical Ballast
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In the context of electrical equipment, a ballast is a device that stabilizes the flow of current. One example occurs in fluorescent lights, which use ballasts to control the flow of electricity through the bulbs.
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