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Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

ODP

See open drip-proof
Oersted
Oersted, Hans Christian (1777-1851) Born: Rudkobing, Langeland, Denmark

is best known as the discoverer of electromagnetism. Oersted was a strong teacher and did much to bring Danish science up to world-class standards. Oersted predicted the magnetic effect of electric current in 1813, but was unable to prove it until 1820. The publication of his results spurred the work of Faraday and Ampere. Oerstad went on to make other contributions in other sciences. He did not, however, return to his study of electricity.
off-axis illumination
illumination that has no on-axis component, i.e., that has no light which is normally incident on the mask. Examples of off-axis illumination include annular and quadrupole illumination.
Ohm
Ohm, Georg Simon (1789-1854) Born: Erlangen, Germany

is best known for his discovery of what we now call Ohm's Law. Ohm held a variety of teaching posts at secondary schools
as well as universities. In 1827 he published his greatest work, Die Galvanische Kette. Along with Andre Ampere, Ohm was the first to publish rigorously mathematical and theoretical work on electricity. Ohm's famous law states that current in a resistor is proportional to the applied voltage and inversely proportional to the resistance. Ohm's work was initially scorned because it lacked the experimental evidence. Worldwide acclaim changed Ohm's fortunes several years later.

He is honored by having his name used as the unit of resistance, the ohm, and the unit of conductivity, the mho.
Ohm's Law
a fundamental law which states that the voltage across a resistance is directly proportional to the current flowing through it. The constant of proportionality is known as the resistance.

This concept can be generalized to include the relationship between the voltage and current in all situations, including alternating voltages and currents. In this case, all the quantities are measured as complex numbers, known as phasors, that are functions of frequency. This broadens the basic definition of resistance, which is a real number measured in ohms, to that of impedance, which is a complex number with magnitude measured in ohms and phase angle in degrees. The real part of the complex number representing impedance is the resistance while the imaginary part is the reactance. Ohm's Law is a central concept to most electrical engineering theories.
ohmic contact
a heavily doped and/or low barrier height metal to semiconductor interface or contact that has a very low resistance relative to the remainder of the device, such that the device performance is not significantly degraded. At lower doping levels, the ohmic contact is described by Ohm's Law, while at higher doping levels, tunneling dominates.
ohmic loss
a term used to describe the power dissipated due to the finite conductivity of the metallic structure of an antenna, waveguide, transmission line, etc.
ohmic medium
a medium in which conductivity is independent of the applied field.
oil circuit breaker
a power circuit breaker that uses oil as an insulating and arc-clearing medium.
oil-filled transformer
a transformer in which the magnetic core and the windings are submerged in an insulating oil. In addition to serving as an insulator, the oil provides a heat exchange medium to cool the transformer.
oil-paper insulation
an insulation scheme used in transformers and cables in which conductors are insulated with heavy paper impregnated with a dielectric oil.
one-line diagram
an abbreviated schematic representation of a power system in which three-phase transmission lines are shown as single lines between principal circuit components and from which circuit parameters are often omitted.
open drip-proof (ODP)
pertaining to a ventilated machine whose openings are constructed to prevent drops of liquid or solid particles falling on the machines at an angle less than 15. from the vertical from entering the machine either directly or by rolling along a horizontal or inwardly inclined surface of the machine.
open-circuit test
a transformer test conducted by applying nominal voltage on the low voltage side while keeping the high voltage side open. By measuring the power in, current, and voltage, the magnetizing reactance of the transformer equivalent circuit can be determined.
open-delta transformer
a connection similar to a delta-delta connection, except that one single-phase transformer is removed. It is used to deliver three-phase power using only two single-phase transformers. The normal capacity of the open-delta transformer is reduced to 57.7% of its delta rating.
optical fiber
a single optical transmission fiber usually comprised of a cylindrical core (5-100 mm diameter) in which the light is guided of higher index of refraction surrounded concentrically by a cladding (125-250 mm diameter) with a lower index of refraction. More properly defined as an optical waveguide. Some optical fibers may have multiple concentric cores and/or claddings.

Optical fibers made be of all glass, all plastic, or a combination of glass core and plastic cladding construction. Optical glass fibers may be silica-or fluoride-based glass.
optical fiber signal distortion
a change in the temporal shape of an optical signal transmitted through an optical fiber caused by a combination of wavelength effects (dispersion) and multimode and polarization effects. The wavelength effects include material dispersion, profile dispersion, and waveguide dispersion. The multi-mode effects cause distortion by the differential time delays between the various modes propagating in a multimode fiber. The polarization effect causes distortion by the differential time delay between the two polarizations of a single mode.
Orange book

See IEEE Color Books
organic light emitting diode
a group of recently developed organic material that emits light in response to electrical input. Although lower in efficiency, they have greater manufacturing flexibility than semiconductor LED.
oscillograph
a continuous recording of the waveforms of an electric power line, formerly made with a cathode-ray tube but currently with a digital signal recorder, kept updated to record abnormalities during switching operations and fault conditions.