1-21 DUMMY ANTENNA—See DUMMY LOAD [16]. DUMMY LOAD—A dissipative but nonradiating device that has the impedance characteristics of an antenna or transmission line. Also called ARTIFICIAL LOAD [11] [16] [17]. DUPLEXER—A radar device that switches the antenna from the transmitter to the receiver and vice versa [18]. DUTY CYCLE—In a transmitter, ratio of time on to time off [12] [18]. DYNAMIC MICROPHONE—A device in which sound waves move a coil of fine wire that is mounted on the back of a diaphragm and located in the magnetic field of a permanent magnet [12]. ECHO—(1) The reflection of the original sound wave as it bounces off a distant surface [10]. (2) The RF signal reflected back from a radar target [18]. ECHO BOX—A resonant cavity device that is used to check the overall performance of a radar system. It receives a portion of the transmitted pulse and retransmits it back to the receiver as a slowly decaying transient [18]. ECLIPSE—A condition in which the satellite is not in view or in direct line of sight with the sun. This happens when the earth is between them [17]. EDDY CURRENT—Induced circulating currents in a conducting material that are caused by a varying magnetic field [2] [5]. EDDY CURRENT LOSS—Losses caused by random current flowing in the core of a transformer. Power is lost in the form of heat [2]. EDISON EFFECT—Also called RICHARDSON EFFECT. The phenomenon wherein electrons emitted from a heated element within a vacuum tube will flow to a second element that is connected to a positive potential [6]. EFFECTIVE VALUE—Same as ROOT-MEAN-SQUARE [2]. EFFICIENCY—The ratio of output-signal power compared to the total input power, generally expressed as a percentage [1] [7]. E-FIELD—Electric field that exists when a difference in electrical potential causes a stress in the dielectric between two points [11]. ELASTICITY—The ability of a substance to return to its original state [10]. ELECTRIC CURRENT—The flow of electrons [1]. ELECTRIC (E) FIELD—The field of force that is produced as a result of a voltage charge on a conductor or antenna [10] [11]. ELECTRICAL CHARGE—Symbol Q, q. Electric energy stored on or in an object. The negative charge is caused by an excess of electrons; the positive charge is caused by a deficiency of electrons [1]. ELECTRICAL CHEMICAL—The action of converting chemical energy into electrical energy [1]. ELECTRICAL-LOCK—A synchro zeroing method. This method is used only when the rotors of the synchros to be zeroed are free to turn and their leads are accessible [15].