FPD (freezing point depressant). A chemical compound added to a liquid to lower its freezing temperature. FPDs are used in the fluid sprayed on an aircraft to deice it before flight.
fractional distillation. The procedure used for separating the various components from a physical mixture of liquids. Crude oil is a mixture of many different types of hydrocarbon fuels which can be separated by carefully raising its temperature. The first products to be released, those having the lowest boiling points, are some of the gaseous fuels; then gasoline, kerosine, diesel fuel, heavy fuel oils, lubricating oils, and finally, tar and asphalt.
fractus (meteorology). Clouds in the form of ragged, irregular shreds, appearing as if they were torn. Fractus clouds of the cumulus family are called fractocumulus, and those in the stratus family are called fractostratus.
francium. An extremely unstable, radioactive chemical element. Francium’s symbol is Fr, and its atomic number is 87. The most stable of francium’s isotopes is Fr 223, which has a half-life of 21 minutes.
frangible. Breakable, or easily broken.
free balloon. A lighter-than-air device that is free to rise in the air. A free balloon is used in weather observations to find the height of the base of the lower layer of clouds. A rubber balloon is filled with helium until it has a specified diameter, and it is released into the air.
The rate of rise of this balloon is known, and the time between its release and its disappearance into the clouds is measured. This time determines the height of the base of the clouds.
free electrons. Electrons that are not bound to any particular atom. They are free to move about from one atom to another in a material. A semiconductor material having a large number of free electrons is called an N-type material.
free fit (threaded fastener). A loose fit between moving parts, used when accuracy of movement is not important. A nut that turns easily over the threads of a screw or bolt is said to have a free fit.
free-running multivibrator (electronic oscillator). A form of oscillator that produces a square-wave output. It uses two transistors arranged in a circuit in such a way that when one transistor begins to conduct (when it turns on), it turns the other transistor off. When the second transistor turns off, it feeds a signal to the first transistor that turns it off. When the first transistor turns off, it turns the second one on.