Ballisto-Cardiograph - An instrument for recording gross mechanical movement of the body as a whole resulting from movements of the heart and blood.

Balloon - A lighter-than-air aircraft consisting essentially of an envelope containing a light gas.

Balmer Series - A series in the line spectrum of the hydrogen atom.

Band Head - In molecular spectra: that edge of a spectral band which is sharply defined.

Band-pass Filter - An optical or electrical filter which transmits a single band of frequencies.

Band Theory or Solids - A theory, associated with the collective electron theory of solids, which postulated the existence of zones or bands of allowed electron energy levels.

Band Width - Of an electrical device or circuit: the range of frequency within the performance with respect to a given characterisitic (e.g. the gain of a receiver or amplifier) falls between specified limits.

Bar - A unit of pressure, defined as106  dyne/cm² (l05N/m²). A more convenient unit is the millibar. One standard atmosphere is, by definiton, 1013.25 mbar precisely.

Barkhausen Effect - The occurrence of a series of discontinuous steps (Barkhausen discontinuities) in the magnetization of a ferromagnetic material as the magnetizing field is changed continuously.

Barn - A unit of area used in expressing a nuclear cross-section. It is equal to 10-24 cm² (10-28 m²) This value was chosen since the cross-sections for all processes for most nuclei lie within the range 10-19 to 10-27 cm².

Barograph - A recording barometer, usually of the aneroid type.

Barometer - An instrument for measuring the pressure of the atmosphere. It can either be of the absolute type, in which the pressure is balanced directly against a column of mercury, or of the indirect type, in which the pressure is measured by the deflection it produces on a suitable elastic struture.

Barometer, Aneroid - A barometer depending forits action on the response of an evacuated capsule of thin corrugated metal to changes in atmospheric pressure.

Barometer, Fortin - A mercury barometer in which the mercury level in the reservoir is always brought to the same height before a reading of the main column is taken, thus permitting the use of a fixed scale.

Barostat - A device by means ofwhich the pressure in an enclosure can be kept at a constant value.

Barostat - A device for ensuring that the current in the circuit to which it is connected remains substantially constant over a wide range of voltage.

Barrier Layer - A localized variation in electrostatic potential between two adjoining regions in which the potential is relatively uniform. which may be thought ofas an electrical double layer.

Barrier, Nuclear: Barrier,Potential: Barrier, Gamow - The region of high potential energy through which a charged particle must pass on leaving or entering an atomic nucleus..

Barrier Width - the width of the barrier layer formed at the boundary between two conductors.

Baryon - A collective term for nucleons and hyperons and sometimes for their anti-particles the anti-baryons.

Base Lines ( Geodesy) - Survey lines, of which the lengths are very accurately measured, used as a basis for subsequent triangulation.

Base Pressure - The suction acting on the base of a moving projectile and giving rise to drag (base drag).
Bathochromic Shift - The shift ofan absorption band toa region of longer wavelength, thus giving visible colour to a previously colourless substance.

Battery - Two or more electrical cells connected and used as a single unit, as in an accumulator.

BAUSCHINGER EFFECT - The phonmenon whereby the mechanical properties ofa metal for tensile loading are differnt from those for compressive loading as a result of its having been plastically deformed in tension and unloaded at a temperature at which it shows appreciable work hardening  d very little recovery properties.

Beam Current - The current represented by a stream ofelectrons in a cathode-ray tube, travelling wave tube, etc. 

Bearing - (1) The horizontal angle between the direction from an observer of an object of interst and a given reference direction. (2) A member used to support, guide or restrain moving elements ofa machine. (3) That part of a beam, grider or similar structure that rests on the support.

Beats - A series of alternate maxima and minima in the amplitude ofvibration produced by the interference of two wave trains having slightly differnt frequencies. The beat frequency is the difference between the two wave frequencies.

Beaufort Notation - A notation of letters, with alternative symbols, used primarily in synoptic meteorology to represent the phenomena  ofweather.

Bel - A logarithmic unit used for expressing power ratios. The number of bels corresponding to a power ratio W is log10 W. It is usually more convenient to use one-tenth of this unit, the decibel.

Bench Marks  (Surveying) - Marks inscribed on a building, pavement