Filter is a device that removes unwanted quantities from the flow of liquids or gases, or from the transmission of electric currents, beams of light, and sound waves. Filters that remove solid particles or other impurities from liquids or gases are made from paper, cloth, charcoal, porcelain, fiberglass, or some other porous material. Glass or gelatin filters are used on cameras to filter out certain light rays (see Photography (Filters)).
Internal-combustion engines use various types of filters to remove impurities from air, lubricating oils, or fuel. Dry-paper filters on carburetors remove impurities from air before it enters the engine. Most oil filters also are made of fibrous paper. Many fuel filters have a stack of ceramic or metal disks separated by narrow spaces, but a few consist of wire screen. Some high-temperature engines also use magnetic filters. The filters attract metallic particles smaller than 1 micron (0.001 millimeter, or 1/25,400 inch).
Cigarette filters, usually made of cellulose acetate, remove some of the tar and nicotine particles from cigarette smoke. Air conditioners use filters made of fiberglass or metal, coated with an adhesive, to remove dust and pollen from the air. Almost all large cities have filtration plants to filter water.
See also Air conditioning (Cleaning the air).
