AI-3 NATIONAL BUREAU OF STANDARDS (NBS)—A bureau of the United States government that is responsible for maintaining the nation's electrical and physical standards. The accuracy of all calibrated test equipment is traceable to NBS through the Navy's METCAL program. OPTICAL TIME-DOMAIN REFLECTOMETER (OTDR)—A piece of test equipment used to test a fiber-optic cable for such things as attenuation, localized losses, and defects. It transmits an optical pulse (usually a laser) into the fiber-optic cable and analyzes the resulting reflections in terms of amplitude versus distance. PARALLAX ERROR—The error in meter readings that results when you look at a meter from some position other than directly in line with the pointer and meter face. A mirror mounted on the meter face aids in eliminating parallax error. PARASITIC—In electronics, an undesirable frequency in an electronic circuit. Usually associated with vacuum-tube amplifiers and oscillators. PERCENT OF MODULATION—In AM signals, the ratio of half the difference between the maximum and minimum amplitudes of a modulated wave to its average amplitude. RF IMPEDANCE BRIDGE—A piece of test equipment used for measuring the combined resistance and reactance of a component, piece of equipment, or system at rf frequencies. SENSITIVITY—In reference to test equipment, the ratio of the response of the test equipment to the magnitude of the measured quantity. Sometimes expressed indirectly by stating the property by which sensitivity is computed (e.g., ohms per volt). STANDARD—An exact value of an electrical quantity (established by international agreement), which serves as a model for measurement of that quantity. STANDING WAVE—The distribution of voltage and current along a transmission line formed by the incident and reflected waves, which has minimum and maximum points on a resultant wave that appears to stand still. STROBOSCOPE—An instrument that allows viewing of rotating or reciprocating objects by producing the optical effect of a slowing or stopping motion. SWEPT-FREQUENCY TESTING—Testing the frequency response of a component or system by applying an rf signal, in which the frequency is varied back and forth through a set frequency range at a steady rate, to the input of a device. The output is then monitored to determine the amplitude of the output with respect to frequency. THERMISTOR—(1) A semiconductor device in which the resistance varies with temperature. (2) A type of bolometer characterized by a decrease in resistance as the dissipated power increases. TIME-DOMAIN REFLECTOMETER—A piece of test equipment used to test a transmission line for defects, such as shorts and opens. It transmits an electrical pulse into the transmission line and analyzes the resulting reflections in terms of emplidute versus distance. TRAIC—A three-terminal device that is similar to two SCRs back-to-back with a common gate and common terminals. Although similar in construction and operation to the SCR, the Triac controls and conducts current flow during both alternations of an ac cycle.