=External Circuit.= (_See_ Circuit, External.) F =F.= The sign commonly employed to designate Fahrenheit. Thus, 30° F. means 30 degrees Fahrenheit, or 30 degrees above zero. =False Magnetic Poles.= (_See_ Magnetic Poles, False.) =Faradic.= Induced current produced from induction-coils and faradic machines. A series of alternating electrostatic discharges, as from a Holtz influence machine. =Faradic Coil.= (_See_ Coil, Faradic.) =Faradic Currents.= (_See_ Currents, Faradic.) =Faradic Machine.= An apparatus designed to produce faradic current. =Feed.= To furnish an electric current, also spoken of in connection with the mechanism that moves the carbons in arc-lamps. =Feeders, or Feed Wires.= The conductors which convey electric currents at different points, as in the trolley system. The current is carried along in large cables strung on poles or laid underground, and at proper distances lines are run in to feed the trolley wire. =Field.= The space in the neighborhood of a dynamo or motor, or other generator of electric current, from which the apparatus takes its electricity, both electrostatic and magnetic. =Field-magnet.= (_See_ Magnet, Field.) =Field of Force.= The space in the neighborhood of an attracting or repelling mass or system. There are two kinds of fields of force--the electro-magnetic and the static--from which the respective pieces of apparatus draw their store of electricity. =Filament.= A long, thin piece of solid substance. It is generally as thin as a thread and flexible enough to be bent. The hairlike element in an incandescent lamp which, when heated by a current, glows and radiates light. =Filaments, Paper.= Filaments for incandescent lamps made of carbonized paper. They were the ones originally used in electric lamps, but have been superseded by other substances easier to handle and more durable. =Flow.= The volume of a current or stream escaping through a conductor, such as a wire, rod or pipe. =Fluorescence.= The property of converting ether waves of one length into waves of another length. The phenomenon is utilized in the production of Geissler tubes and X-rays. =Fluoroscope.= An apparatus for making examinations by means of the X-rays. =Fluoroscopic Screen.= A screen overspread with fluorescent material and employed for fluoroscopic examinations in connection with the X-rays. =Force.= Any change in the condition of matter with respect to motion or rest. Force is measured by the acceleration or change of motion that it can impart to a body of a unit mass in a unit of time. For instance, ten pounds pressure of steam will be indicated on a gauge made for measuring steam. That pressure of steam, with the proper volume behind it, is capable of instantly producing a given part of a horse-power. In the same way ten volts of electro-motive force is capable of pushing a current so as to exert a certain fraction of horse-power.