1-25 EUTECTIC SOLDER—An alloy of 63 percent tin and 37 percent lead. Melts at 361º F [14]. EXCITATION VOLTAGE—The supply voltage required to activate a circuit [15]. EXCITING CURRENT—The current that flows in the primary winding of a transformer when the secondary is open-circuited; it produces a magnetic flux field. Also called magnetizing current [2]. EXCLUSIVE OR—A function whose output is a 1 if one and only one of the input variables is a 1 [13]. EXCLUSIVE-OR GATE—A gate that produces a logic 1 output when the inputs are different, but not when they are the same [13]. EXPONENT—The numeral written in superscript (10^{2}) which indicates the power to which the base is to be raised [13]. EXPRESSION—A validated series of variables, constants, and functions that can be connected by operating symbols to describe a desired computation [13]. EXTERNALLY EXCITED METER—A term used to describe meters that get their power from the circuit to which they are connected [16]. EXTERNALLY SYNCHRONIZED RADAR—A radar system in which timing pulses are generated by a master oscillator external to the transmitter [18]. EXTREMELY HIGH FREQUENCY—The band of frequencies from 30 gigahertz to 300 gigahertz [17]. EXTREMELY LOW FREQUENCY—The band of frequencies up to 300 hertz [17]. EXTRINSIC—A semiconductor in which impurities have been added to create certain charge carrier concentrations [7]. FACSIMILE—The method for transmitting and receiving still images. These images can be maps, photographs, and handwritten or printed text [17]. FACTOR—Any of the elements, quantities, or symbols that, when multiplied together, form a product [13]. FADING—Variations in signal strength by atmospheric conditions [101 [17]. FARAD—The basic unit of capacitance. A capacitor has a capacitance of 1 farad when a voltage potential of 1 volt across it produces a charge of 1 coulomb [2]. FARADAY ROTATION—The rotation of the plane of polarization of electromagnetic energy when it passes a substance influenced by a magnetic field that has a component in the direction of propagation [11]. FAST-TIME-CONSTANT CIRCUIT—Differentiator circuit in the first video amplifier that allows only the leading edges of target returns, no matter how small or large, to be used [18]. FEEDBACK—The return of a portion of the output of a circuit to its input [8] [18]. FEEDER—A transmission line that carries energy to the antenna [10]. FEEDHORN—A horn radiator used to feed a reflector [18]. FEP—A synthetic type of insulation (fluorinated ethylene propylene) [4].