Induction, Mutual - Electromagnetic induction between two circuits,e.g. two parallel wires.

Induction, Self. Electromagnetic induction within a single circuit, e.g. a coil.

Inductometer. A variable inductor, used for varying the inductance in a        network, e.g. for balancing a bridge.

Inductor - (1) A device which has an inductance which is large compared with its resistance and self capacitance. (2) One of the rotating masses of magnetic material used to produce the necessary changes in magnetic flux density in an inductor generator.

Inductor Generator - An electric generator in which the field and armature windings are fixed relative to each other in necessary changes in magnetic flux density being produced by rotating masses of magnetic material.

Inertial Frame of Reference - A frame of reference that remains constant, usually a coordinate system based on the "fixed" stars.

Information Theory - Concens the separation and measurement of information content from other aspects of information such as semantic value. In this theory the concept of entropy has been taken over to signify a measure of uncertainty.

Infrared Radiation - That part of the electromagnetic spectrum which extends from the longwave limit of visible radiation (about 7500Å or 0.75 µm) to a wavelength sometimes taken as 350 µm, since this represents the extreme wavelength studied by conventional heat radiation methods, and sometimes taken to include in addition the microwave range up to 10000 µm or 0.1cm.
Inner Bremsstrahlung: Internal Bremsstrahlung - Bremsstrahlung which may arise from the emission or absorption of a charged particle by an atomic nucleus.

Insolation - (1) Radiation received from the Sun. (2) The energy flux density of this radiation. See also: Radiant flux density.

Insulation - (1) Dielectric material having permanent low electrical conductivity and high dielectric strength, used to separate conducting materials which are at different electrical potentials. (2) Material having low thermal conductivity, used to retain or exclude heat.

Insulator - Refers to a material which does not conduct electricity or heat as the case may be.

Insulator, Electrical - Refers, apart from its general meaning, to an appliance designed to support or locate an uninsulated current-carrying conductor maintained at a different potential from that of its surroundings.

Integrated Circuit - An electronic circuit which is so designed and constructed that it may be considered to be an indivisible whole both for use and servicing. The applications are most numerous in computing, and data processing and control, but linear amplifier circuits are also available. Either thin films or silicon “chips” are used for the circuit elements. Where both are used in the same circuit it is known as a hybrid circuit. .

Intensity - A quantitative measure of the "strength" of a physical effect. For a beam of radiation (heat, light, sound, sub-atomic particles) it is usually taken as the amount of radiant energy incident on or crossing unit area per second, but in other contexts different conceptes are employed, e.g. for electric and magnetic intensity.

Intensive Variable - One which is independent of the amount of matter considered. Thus, temperature, density and pressure are typical intensive variables. They are opposed to extensive variables such as total energy, mass and volume. An intensive variable is also known as an intensive quantity or intensive parameter.
Immersion Lens, Optical - The term usually denotes that the space between the object and objective of a microscope is filled with a medium (usually oil) of higher refractive index than air. This increases the numerical aperture and hence improves the resolution. In an electron lens the term has a different significance.

Impact - The collision between two bodies when relatively large contact forces exist during a very short interval of time.

Impedance. The (complex) ratio of voltage to current in an alternating current circuit. The real part of the impedance is the resistance and the imaginary part is the reactance. Analogous quantities occur in acoustics and mechanics.

Impedance Load - The impedance external to the source in an electric circuit. It is also known as the external impedance.

Impeller. The rotating member of a centrifugal pump or blower, which imparts kinetic energy to the fluid which is being pumped.
Imperial System of Units - A system based on the Imperial Standard Yard as a standard of length and the Imperial Pound as a stand d of mass. The unit of volume, the gallon, is defined in terms of the volume of 10 lb of distilled water under specified conditions. It was developed in Great Britain and adopted, with some modifications, in the U.S.A.

Impulse Approximation - For a collision between a particle and a (bound) target particle: the neglect, for purposes of calculation, of the binding forces on the target particle during the collision.

Impulse Electric Strength - Of an insulator for a voltage applied for a very short time (microseconds): the breakdown voltage divided by the thickness of the insulator.

Impulse. Generator - An arrangement for producing high voltage pulses, usually by charging capacitors in paralle1 and discharging them in series.

Impurity Centre - A foreign atom or group of atoms in a crystal. It often serves to activate a phosphor.

Incoherent - As applied to emitted or scattered waves: implies the absence of a definite phase relationship between two or more sets of waves. Where a definite phase relationship does exist the waves are said to be coherent.

Indeterminacy Principle: Uncertainty Principle - A principle enunciated by Heisenberg which states that no measurement can determine both the position of a particle and its momentum (or any other pair of conjugate variables, such as time and energy) so accurately that the product of their errors is less than Planck's constant. It is a necessary result of the quantum theory.

Indicator Diagram - A graphical representation of the pressure existing in the cylinder of a reciprocating engine for any position of the piston.

Interfacial Tension. The free energy per unit area of the interface between two phases.

Interference - (1) In aerodynamics: the aerodynamic influence of one body upon another. (2) Ina communications system: the disturbance caused by unwanted signals. (3) Of a progressive wave motion: the effects resulting from the superposition of coherent wave trains of the same or nearly the same frequency, as in diffraction, for example: The interference may be constructive or destructive according as the waves are in or out of phase.

Interference Filter - An optical filter which depends on interference in a thin film to give selective transmission of a narrow wave band.

Interference Fringes. A pattern of light and dark bands arising from  the constructive and destructive interference of wave trains.

Interferometer - An instrument employing the interference of light to obtain knowledge of the light itself, as in spectroscopy, or to perform physical measurements with known light, such as metrological measurements or measurements of refractive index. It consists essentially of a beam splitter, which divides the light from some suitable source into. two or more beams, and a recombination system, which brings the beams together again in a condition suitable for interference to occur.

Intermolecular Potential - The work done in separating two molecules by an infinite distance.

Internal Combustion Engine - A heat engine in which a fuel (oil, gas or coal) is burnt with air to form the working fluid. By extension the indirectly heated gas turbine is usually classed as an internal combustion engine.