A. (1) Linear acceleration (a). (2) Mean sound absorption coefficient (a). (3) Element argon (A). (4) Angstrom unit (A or A) . (5) First van der Waals constant (a) . (6) Chemical activity (a). (7) Accommodation coefficient (a). (8) Amplification

of amplifier (A). (9) Amplitude (A). (10) Refracting angle of prism (A). (11) Area (A). (12) Specific rotation of light |a|. (13) Free energy, Helmholtz, which is also known as isothermal work function, total (A), per unit mass (a), per mole (a, A or Am). (14) Factor in Richardson- Dushman equation (A). (15) Width of slit (transparent portion) (a). (16) Atomic weight (A). (17) First Couchy constant (A). (18) Strength of simple acoustic source (A). (19) Magnetic vector potential (A). (20) Bohr radius (ai). (21) Radius of acoustical tube, disc or membrane (a) .

AA+, A. Terminal markings for sources of filament voltages in electronic equipment. (See A supply.)

A BATTERY. Power source for filaments in battery-operated electronic equipment.

A SUPPLY. The source of the heating current for the cathode of an electronic tube. In the early days of radio the various voltages needed to operate a receiver were obtained from batteries, called A, B and C batteries, supplying the filament, plate and grid voltages respectively. These letter designations have carried over to the present-day sources, although the voltages are usually obtained now from an a-c source, either directly as in the case of the A supply or indirectly for B and C voltages.

AB-. A prefix attached to the names of the practical electrical units to indicate the corresponding unit in the cgs electromagnetic system (emu) , e.g., abampere, abvolt

ABAMPERE. The cgs electromagnetic unit of current. It is that current which, when flowing in straight parallel wires 1 cm apart in free space, will produce a force of 2 dynes per cm length on each wire. One abampere is ten amperes.

ABBE CONDENSER. A compound lens used for directing light through the object of a compound microscope. All the light enters the object at an angle with the axis of the microscope.

ABBE NUMBER. The reciprocal of the dispersive power of a material.

ABBE REFRACTOMETER. See refractometer. Abbe.

ABBE SINE CONDITION. The relationship ny sin θ= n'y' sin θ', where n,n' are indices of refraction, y, y' are distances from optical axis, andθ,θ' are angles light rays make with the optical axis. A failure of an optical surface to satisfy the sine condition is a measure of the coma of the surface.

ABBE THEORYOF THE RESOLUTIONOF A MICROSCOPE. A theory relating the resolution of the instrument to the wavelength of the light and the aperture of the instrument.

ABELIAN GROUP. A commutative group, thus AB = BA where A, B are any two elements contained in it. A simple example is the cyclic group of order n.

ABERRATION, ANGLE OF. See aberration of light, Bradley.

ABERRATION(S), FIVE GEOMETRICAL. (1) Spherical Aberration. (2) Coma. (3) Astigmatism. (4) Curvature of Field. (5) Distortion. Also called the "third-order" aberrations and first comprehensively analysed by Von Seidel.

ABERRATION, LEAST CIRCLE OF. The area of minimum cross section of the rays from an optical system with spherical aberration.

ABERRATION OF LIGHT (BRADLEY). The apparent displacement of a star due to the motion of the earth in its orbit. Maximum value about 20.5 seconds of arc when the star is viewed normal to the velocity of the earth. Distinct from parallax.

ABERRATION, OPTICAL. The failure of an optical system to form an image of a point as a point, of a straight line as a straight line, and of an angle as an equal angle. (See spherical aberration, astigmatism, coma, curvature of field, distortion (of the image), and chromatic aberration.)

ABNEY COLORIMETER. See colorimeter, Abney.

ABNEY EFFECT. A shift in hue which is the result of a variation in purity and, therefore, in saturation. The Abney effect may be represented by chromaticity loci, of specified luminance, with the hue and brightness constant, when purity and, therefore, saturation are varied. It is a relationship, of psychophysical nature, between psychophysical specifications and color sensation attributes.

ABNEY MOUNTING. A method for mounting a grating, plateholder and slit on a Rowland circle and moving only the slit to observe different parts of the spectrum.

ABNORMAL GLOW. In a glow-discharge device, the flow of current equal or greater than the magnitude which causes the cathode to be completely covered with glow.

ABNORMAL REFLECTIONS. Ionospheric reflections of radiowaves at frequencies higher than the critical frequency of the layer. Sometimes referred to as sporadic reflections.

ABRAHAM THEORY OF THE ELECTRON. Model of the electron as a rigid spherical ball of charge, the mass being regarded as of purely electromagnetic origin (1903). Yields an incorrect expression for the variation of mass with velocity, and abandoned when the predictions of special relativity theory (see relativity theory, special) were confirmed.

ABSCISSA. The horizontal coordinate of a point in a two-dimensional system, commonly rectangular Cartesian, and usually designated by x. Together with the ordinate it locates the position of the point in a plane.

ABSOLUTE FUTURE OF AN EVENT. All events which could be reached by a signal emitted at the event and moving with velocity less than or equal to that of light in a vacuum.

ABSOLUTE HUMIDITY. See humidity, absolute.

ABSOLUTE PAST OF AN EVENT. All events from which a signal, moving with velocity less than or equal to that of light in a vacuum, could be emitted to reach the event in question.

ABSOLUTE SPACE-TIME. A fundamental concept underlying Newtonian mechanics is that there exists a preferred reference system to which all measurements should be referred. This is known as absolute spacetime. The assumption of such a system IB replaced in relativistic mechanics by the principle of equivalence. (See equivalence, principle of.)

ABSOLUTE TEMPERATURE SCALE. See temperature scale, absolute.

ABSOLUTE UNITS. Any set of units defined in terms of fundamental (arbitrary) units of mass, length, and time by connecting physical equations. Compare international units. (Cgs electrostatic, cgs electromagnetic, and MKSA units are absolute units.)

ABSOLUTE ZERO. The temperature at which a system would undergo a reversible isothermal process without transfer of heat This is the temperature at which the volume of an ideal gas would become zero. The value calculated from the limiting value of the coefficient of expansion of various real gases is ~273.160 C.

ABSORPTION. (1) The process whereby the total number of particles emerging from a body of matter is reduced relative to the number entering, as a result of interaction of the particles with the body. (2) The process whereby the kinetic energy of a particle is reduced while traversing a body of matter. This loss of kinetic

energy of corpuscular radiation is also referred to as moderation, slowing, or stopping. (3) The process whereby some or all of the energy of sound waves or electromagnetic radiations is transferred to the substance on which they are incident or which they traverse. (4) The process of "attraction into the mass" of one substance by another so that the absorbed substance disappears physically.

ABSORPTION BAND. A region of the absorption spectrum in which the absorptivity passes through a maximum or inflection.

ABSORPTION CELL. A glass vessel used to hold liquids for the determination of their absorption spectra.

ABSORPTION COEFFICIENT. (1) For the absorption of one substance or phase in another, as in the absorption of a gas in a liquid, the absorption coefficient is the volume of gas dissolved by a specified volume of solvent; thus a widely-used coefficient is the quantity a in the expression α = Vo/Vp, where F is the volume of gas reduced to standard conditions, V is the volume of liquid and p is the partial pressure of the gas. (2) In the case of sound, the absorption coefficient (which is also called the acoustical absorptivity) is defined as the fraction of the incident sound energy absorbed by a surface or medium, the surface being considered part of an infinite area. (3) In the most general use of the term absorption coefficient, applied to electromagnetic radiation and atomic and sub-atomic particles, it is a measure of the rate of decrease in intensity of a beam of photons or particles in its passage through a particular substance. One complication in the statement of the absorption coefficient arises from the cause of the decrease in intensity. When light, x-rays, or other electromagnetic radiation enters a body of

matter, it experiences in general two types of attenuation. Part of it is subjected to scattering, being reflected in all directions, while another portion is absorbed by being converted into other forms of energy. The scattered radiation may still be effective in the same ways as the original, but the absorbed portion ceases to exist as radiation or is re-emitted as secondary radiation. Strictly therefore, we have to distinguish the true absorption coefficient from the scattering coefficient; but for practical purposes it is sometimes convenient to add them together as the total attenuation or extinction coefficient.

Accurate measurements upon radiation which has traversed various thicknesses of matter has established that any infinitelythin layer perpendicular to the direction of propagation cuts down the flux density by a fraction of its value proportional to the thickness of the layer, whence by integration(when permissible) the flux density after having penetrated the medium to a distance x is

I=Ioe−ax

in which I0 is the flux density just after entrance into the medium (i.e. for x = 0) . (See the Bouguer law.) For true absorption, the constant a is the absorption coefficient. For scattering, which obeys the same law, a is the scattering coefficient. And for the total attenuation, including both, it is the extinction coefficient, which is the sum of the absorption and the scattering coefficients.

The absorption coefficient may be computed for total radiation which enters the absorbing material, for the visible luminous radiation or as a function of wavelength, being in that

case, the spectral absorption coefficient The absorption coefficient divided by the density of the absorbing medium is called the mass absorption coefficient. (See absorption coefficient, mass and other qualified terms.)

ABSORPTION COEFFICIENT, ATOMIC. The atomic absorption coefficient of an element is the fractional decrease in intensity, per number of atoms per unit area; it is equal to the linear absorption coefficient (see absorption coefficient, linear) divided by the number of atoms per unit volume, or to the mass absorption coefficient (see absorption coefficient, mass) divided by the number of atoms per unit mass. If the medium consists of only one nuclide, the atomic absorption coefficient μα is equivalent to the total cross section for the radiation in question.

ABSORPTION COEFFICIENT, MASS. The mass absorption coefficient μmis the fractional decrease in intensity per unit surface density. For a substance of densityρ, μm is equal to ρ, and hence is independent of the density.

ABSORPTION CURVE. The graphical relationship between thickness of absorbing material and intensity of transmitted radiation.

ABSORPTION DISCONTINUITY. A discontinuity appearing in the absorption coefficient of a substance for a particular type of radiation when expressed as a function of the energy (or frequency or wavelength) of this radiation. An absorption discontinuity is often associated with anomalies in other variables such as the refractive index.

ABSORPTION EDGE. The wavelength corresponding to an abrupt discontinuity in the intensity of an absorption spectrum, notably an x-ray absorption spectrum, which gives the appearance of a sharp edge in the photograph of such a spectrum.

ABSORPTION FACTOR. In any absorbing system, especially in the case of absorption of radiation,, the ratio of the total unabsorbed radiation to the total incident radiation, or to the total radiation transmitted in the absence of the absorbing substance. (Cf. absorptivity.)

ABSORPTION LIMIT. See absorption discontinuity.

ABSORPTION LOSS, ACOUSTIC. That part of the transmission loss due to the dissipation or conversion of sound energy into other forms of energy (e.g., heat), either within the medium or attendant upon a reflection.

ABSORPTION MESH. A filter element used in a waveguide system to absorb spurious components of electromagnetic energy.

ABSORPTION MODULATION. See modulation, absorption.

ABSORPTION, SELECTIVE. Absorption which varies in amount with wavelength.

ABSORPTION SPECTRUM. See spectrum, absorption.

ABSORPTION TRAP. See trap.

ABSORPTIVE POWER, OPTICAL. The same as absorptivity.

ABSORPTIVITY, OPTICAL. The transmissivity subtracted from unity.

ABUNDANCE RATIO. The proportions of the various isotopes making up a particular specimen of an element.

ABVOLT, The cgs electromagnetic unit of potential difference and electromotive force. It is the potential difference that must exist between two points in order that one erg of work be done when one abcoulomb of charge is moved from one point to the other. One abvolt is 10-8 volt.

ACCELERATION. The time rate of change of velocity. Like velocity, acceleration is a vector quantity, requiring the specification of both a magnitude and a direction. The defining equation is

a=d vdt

where v is the instantaneous velocity and t the time. Acceleration may be indicative of a change in speed, of a change in the direction of a velocity of constant magnitude, or of a combination of the two. The quantity just defined is strictly speaking the instantaneous acceleration. (Cf. acceleration, average.)

ACCELERATION, ANGULAR. Angular acceleration is the time rate of change of the angular velocity, expressed by the vector derivative dω /dt. Only in case the direction of the axis remains unchanged can the angular velocity and angular acceleration be treated as scalars. The effect of torque applied to a body free to rotate about an axis is to give it angular acceleration, and the

opposition offered by the body to this process gives rise to the concept of moment of inertia.

ACCELERATION, AVERAGE. If the instantaneous velocity of a particle is V1 at a given instant and V2 at a time Δt later, the average acceleration during the time Δt is defined as:

a=v2−v1

Δt

ACCELERATION, INSTANTANEOUS. See acceleration.

ACCELERATION OF GRAVITY. (1) The ratio of the weight of a material particle to its mass at any specific point in an approximately uniform gravitational field. This is the acceleration with which a body would fall in the absence of all other disturbing forces, such as those due to friction, (2) Specifically, the acceleration with which a body falls in vacuo at a given point on or near a given point on the earth's surface. This acceleration, frequently denoted by g, varies by less than one percent over the entire surface of the earth. Its "average value77 has been defined by the International Commission of Weights and Measures as 9.80665 M/S2 or 32.174 ft/S2 . Its value at the poles is 9.8321 M/S2 and at the equator 9.7799 M/S2.

ACCELERATOR, CONSTANT - POTENTIAL. A device in which a d-c potential is applied to an accelerating tube to produce high-energy ions or electrons.

ACCELEROMETER. (1) An instrument for determining the acceleration of the system with which it moves. (2) A transducer

which gives an indication, usually in the form of a voltage proportional to the acceleration to which it is subjected.

ACCEPTOR (IN A SEMICONDUCTOR). See acceptor impurity.

ACCEPTOR ENERGY LEVEL. An acceptor impurity atom in a crystal is equivalent to an excess negative charge, since its atomic core is insufficiently charged to neutralize its share of the covalent bonding electrons. Consequently, it can attract a positive charge, such as a hole in the electron distribution, forming bound states which lie just above the top of the valence band. The promotion of an electron to one of these levels frees a hole for conduction, as in a p-type semiconductor (see semiconductor, p-type).

ACCURACY. The quality of correctness or freedom from error. Distinguished from precision as in the examples: (a) ". . . this procedure measures the precision (reproducibility) of the test, not its accuracy (closeness to the true value)." (b) A four-place table correctly computed is more accurate but less precise than a six-place table containing errors, (c) The accuracy of an instrument is a number or quantity which defines its limit of error. (See also precision.) The actual error in measurement can seldom be determined, but its magnitude may usually be estimated.

ACTIVATION. (1) In nuclear physics, the process of inducing radioactivity through neutron bombardment or by other types of radiation (see cross section, activation). (2) In electron-tube technology, the process by which the cathode is treated in order that maximum emission may occur. (3) The transfer of a sufficient quantity of energy to an atomic system to raise it to an excited state

in which it can participate in a process not possible when the system is in its ground state. (See activation energy.)

ACTIVATION ENERGY. The excess energy over the ground state which must be acquired by an atomic system in order that a particular process may occur. Examples are: the energy needed by a molecule to take part in a chemical reaction, by an electron to reach the conduction band in a semiconductor, by a lattice defect to move a neighboring site.

ACTIVATOR. An impurity atom present in a solid and making possible the effects of luminescence, or markedly increasing their efficiency. Examples are copper in zinc sulfide, and thallium in potassium chloride.

ACTIVE. This word is used in nuclear physics for three special meanings: (1) Fissionable (active material). (2) Radioactive (active sample). (3) The active part of a reactor is the core (active lattice) .

ADHESION AND COHESION. In physics, the terms adhesion and cohesion designate intermolecular forces holding matter together. The tendency of matter to hold itself together or to cling to other matter is one of its most characteristic properties. Adhesion and cohesion are merely different aspects of the same phenomenon, which is apparently of the nature of an intermolecular attraction. We speak of cohesion as an interaction between adjacent parts of the same body and as acting throughout the interior of its substance, while adhesion refers to a similar interaction between the closely contiguous surfaces of adjacent bodies.

There is reason to believe that as two neutral molecules or atoms approach each other, their mutual potential energy reaches a minimum value at a certain equilibrium distance; so that work would be necessary either to push them closer or to pull them farther apart, because of forces which are probably electrical. The distribution of molecules, ions, or atoms in a solid is determined by this type of equilibrium, and the regular spacing of crystal structure and the architecture of the molecule itself are dependent upon it. Any force tending to diminish the equilibrium distance meets with the rapidly increasing reaction of compressive elasticity, while any force tending to increase it is opposed by cohesion, which increases at first and then rapidly diminishes toward zero as the point of fracture is reached.

The behavior of bodies which are aggregates of crystals or of fibers is complicated by the friction and the adhesion of the adjacent particles, so that the ultimate strength of a material is not a safe measure of its true cohesion. A filament of spun quartz may be much stronger when freshly drawn than later when crystallization replaces its initial cohesion by the adhesion between separate crystals; and yarn is not nearly so strong as the cotton or wool fiber composing it.

Adhesion increases with closeness of contact. This explains why one must bear down with a pencil to make a mark on paper, why fine dust adheres more firmly than coarse sand, and why a liquid or a gum usually sticks to a solid better than another solid does. Cohesion in liquids is usually less, and in gases it is always much less, than in solids. Aside from the pressure in liquids due to external causes, there is presumably a very great internal or intrinsic

pressure, due to intermolecular attraction, but not capable of direct measurement by means at our disposal. The clearest evidences of its existence are the work required for thermal expansion and the phenomenon of surface tension.

ADIABAT. If a thermally-isolated system moves through a series of equilibrium states, the locus of the points representing these states on a graph is called an adiabat. Thus in meteorology, an adiabat is a line on a chart showing the adiabatic lapse rate (wet or dry).

ADIABATIC. Occurring without change in heat content, i.e., without gain or loss of heat by the system involved.

ADSORPTION. A type of adhesion which takes place at the surface of a solid or a liquid In contact with another medium, resulting in an accumulation or increased concentration of molecules or other particles from that medium in the immediate vicinity of the surface

ADSORPTION, ACTIVATED. Chemisorption (adsorption involving forces of chemical nature) in which the activation energy is relatively high. (See also adsorption, types of.)

ADVECTION. The transfer of air and air characteristics by horizontal motion. Fog drifts from one place to another by advection. Cold air moves from polar regions southward. Large-scale north-south advection is more prominent in the northern hemisphere than the southern, but west-to-east advection is prominent on both sides of the equator.

AERODYNAMICS. A phase of the mechanics of fluids, its study being limited to the reactions caused by relative motion between the fluid and solid, with the fluid being air. Sometimes this strict definition is broadened so that aerodynamics may also include the reactions of gases other than air. The scope of the subject of aerodynamics is, nevertheless, broad. It encompasses the flow of gases in conduits, the effects of winds on static structures such as building, chimneys, and bridges, and the effect on moving bodies of the atmosphere through which they move.

AEROSOL. A colloidal system in which a gas, usually air, is the continuous medium, and particles of solid or liquid are dispersed in it.

AEROSTATICS. The science of gases at rest (mechanical equilibrium). (Cf. aerodynamics, the science of gases in motion.)

AIR MASSES. Very large parcels of air ranging from about 500-5000 miles in lateral dimensions and from several thousand feet to several miles deep, which have properties (temperature, humidity, thermal structure) that vary only slightly, or vary linearly, from point to point within the parcel. Air masses develop over large, relatively-homogeneous geographical areas where air is stagnant for a sufficient period to acquire the characteristics of that region. These regions are either continental or maritime and are known as airmass source regions. After an air mass begins to move from its source region it acquires modifying features characteristic of the surface over which it travels. Modification continues until the air mass loses its identity in the general atmospheric circulation. Classification of air masses begins from latitudinal consideration. There are four

major zones which contribute primary classifications: (1) Arctic, (2) Polar, (3) Tropical, (4) Equatorial. These are subdivided into Maritime (m) and Continental (c), depending upon the exact source region. Finally each air mass must be classified as either cold (k) or warm (w) . A cold air mass is one which is colder than the surface over which it is traveling and is therefore being heated from below. A warm air mass is one which is warmer than the surface over which it is traveling and is therefore being cooled from below.

ALPHA (α). (1) Angular acceleration (α). (2) Plane angle (α). (3) Linear expansion, or coefficient of linear expansion (α). (4) Current amplification in transistors (α). (5) In nuclear reactor theory, (α) the reciprocal asymptotic period, or (b) the ratio, in a fissionable element, of neutron radiative capture (see capture radiative (2)) to fission cross-section (α). (6) Thermal diffusibility (α) .(7) Optical aperture (α). (8) Attenuation constant (α). (9) Fine structure or Sommerfeld constant (α). (10) Thermal coefficient of resistance (α). (11) Most probable speed (α). (12) Degree of electrolytic dissociation (α). (13) Coefficient of recombination (α). (14) See alpha-particle and alpha-ray. (15) Plane angle (α). (16) Angular resolving power of telescope (α). (17) Half-angle subtended at point object by objective of microscope (α). (18) Coefficient of recombination (α). (19) Specific rotation (α). (20) Current amplification (α).

AMPERE. A unit of electrical current, abbrev. as A or amp. (1) The absolute ampere is exactly one-tenth of an abampere, the electromagnetic unit (emu) of current The absolute ampere has been the legal standard of current since 1950. (2) The International ampere, the legal standard prior to 1950, is the steady current which

must flow across a surface in order that one International coulomb of electricity shall pass the surface during each second. 1 Int. amp = 0.999835 Abs. amp.

AMPERE LAW. This is a classic law of electromagnetism, useful in discussions of electrodynamics. It has been stated in two apparently distinct forms, which are, however, interconvertible. One form, sometimes known as the Laplace law, states that the electric current i (abamperes), flowing along any line through an element of length dl, gives rise, at a point P distant r (cm) from the element, to a magnetic field of intensity dH = ipdl/r 3(oersteds), in which p is the perpendicular distance from P to the line of the element dl', or dH = i sin θdl/r2 , in which θ is the angle between the line of the element and the line joining dl to P. (See vector potential.) The ultimate basis of the law is, of course, experimental It may, for example, be deduced indirectly from the Biot-Savart law for the field about an infinitely long, straight wire. Ampere's law furnishes a basis for the solution of all problems relating to the magnetic fields produced by electric currents. What is sometimes called the circuital form of Ampere's law may be tangibly expressed by saying that if a unit magnetic pole is carried completely around a conductor or system of conductors in which electricity is flowing, in such a way as to oppose the field set up by the currents, the work done, in ergs, is 4π times the algebraic sum of the currents, in abamperes. This is easily illustrated by a special case. The Biot-Savart law above referred to gives, as the magnetic field at a point distant r (cm) from an infinitely long, straight wire carrying a current i (abamperes), the value H = 2i/r (oersteds), directed, of course, along the circumference of the circle having r as its radius. The force acting

upon a unit pole placed at this point is therefore 2i/r (dynes). If now the pole is moved around the circle, against the field, the work done is

2ir

x 2 πr=4 πi

Maxwell pointed out that Ampere's law holds only for constant currents, and that when currents vary, the resulting changes of electric displacement in the surrounding space, giving rise to the radiation of energy in the form of electromagnetic waves, involves modifications embodied in the so-called Maxwell- Ampere law as expressed by the first of Maxwell's equations. The differential form of Ampere's law cannot be experimentally verified; we can observe only the field due to a closed current loop, not from isolated elements. Experimentally, we can verify that the result of integrating the Ampere law over any closed circuit gives the correct result.

AMPLITUDE. (1) If a complex number is represented in polar coordinates it becomes r(cos + i sinθ) and the angle θ is the amplitude, argument, or phase of the number. (See de Moivre theorem.) The term is also used to designate a parameter occurring in elliptic functions and integrals. (2) The crest or maximum value of a periodic (or specifically H simple harmonic function of space or time) or, more generally, any parameter that when changed, merely represents a change in scale factor. In amplitude-molulation systems, this quantity becomes a function of time, and its instantaneous value is of importance, but it is still referred to as the amplitude.

ANGLE. The figure obtained by drawing two straight lines from a point. In trigonometry, an angle measures the rotation of one straight

line, the terminal line, about a fixed point on an initial line. It is positive if the direction of rotation is counterclockwise. If the magnitude of the angle equals 2π radians, it is called a perigon angle and such an angle, divided into 360 equal parts, has a magnitude of 360 degrees (360). A right angle equals 90 or π/2 radians; a straight angle, 180 or π radians; an acute angle is less than 90 ; an obtuse angle, greater than 90 (but frequently limited to one less than 180). (See also Euler angles; angle, dihedral; angle, solid; triangle, spherical.)

ANNIHILATION. (1) A process in which a pair of anti-particles meet and convert spontaneously into one or more photons; it is the inverse of pair production. The commonest example is the annihilation of an electron and a positron, the rest masses of which are converted into (usually) two 0.511-mev photons according to the principle of mass-energy equivalence. (2) The conversion of rest mass into electromagnetic radiation.

ANODE. The electrode via which current enters a device. The anode is the positive terminal of an electroplating cell, but the negative terminal of a battery. The term anode is customarily used for any thermionic tube electrode operated at an appreciably-positive potential.

ANTENNA. In the process of radio communication the power generated in the transmitter must be projected or radiated into space and at the receiver some of this radiated energy must be abstracted from the passing radio wave and fed into the receiver proper. It is the antenna wrhich radiates the power at the transmitter and which picks up the signal at the receiver. The antenna form ranges from a

simple short length of wire for the receiver to an elaborate array of wires or steel towers for large transmitters. When alternating voltage of a high frequency is connected to a conductor which is open at the end a corresponding high-frequency a-c will flow in the conductor and return to the voltage source through the capacitance between the conductor and the rest of the circuit. This rapid a-c causes energy to be radiated into space from the conductor. This energy travels out from the conductor and does not return. The conductor in this case is the antenna (of course the various connecting wires of the transmitter also have high-frequency a-c and hence will radiate to some extent but very inefficiently) . The efficiency with which an antenna radiates is determined by its length and configuration, and its location with respect to the ground, surrounding objects, etc. In general, better radiation is obtained when the antenna length is an appreciable part of a wavelength of the radio signal. Thus they are usually such values as quarter-wave, half-wave, etc.

ANTINEUTRINO. (1) In the most frequent usage, a particle whose emission is postulated to accompany radioactive decay by positron emission or electron capture. The antineutrmo may be looked upon as a hole in the negative-energy Dirac sea of neutrinos. Since there is no possibility of charge differentiation between the antineutrino and the neutrino, differentiation between these two particles can be made only on the basis of such properties as the sign of the ratio of magnetic moment to angular momentum. Neither the neutrino nor the antineutrino has been detected. (2) In a less common usage the terms neutrino and antineutrino are used in reverse sense to that stated above, i.e., the neutrino is said to accompany positron emission and the antineutrino, electron emission

ARCHIMEDES PRINCIPLE. A body immersed in a fluid is acted on by a buoyancy force, made evident by a loss of weight, equal to the weight of displaced fluid.

ASSOCIATIVE LAW. Addition and multiplication processes obey this law if the following relations hold:

a + (b + c) = (a + b) + c;

(ab)c = a(bc) = abc.

ASTIGMATISM. (1) A defect in a lens, including the lens of the eye, in which there is a difference in the radius of curvature of the lens as observed in one plane from that observed in another plane. (2) An aberration of a lens with spherical surfaces such that the image of a point not lying on the optical axis is a pair of short lines normal to each other and at slightly different distances from the lens. (3) In an electron-beam tube, a focus defect in which the electrons in different axial planes come to focus at different points.

ASYMMETRIC. Not symmetric. The term is generally applied only to functions or systems which are neither symmetric nor antisymmetric.

ATMOSPHERE. (1) A gaseous envelope surrounding a body, or a mass of gas occupying a region. (2) A standard unit of pressure (see atmosphere, standard). (3) The cluster of impurity atoms formed around a dislocation line, and responsible for Cottrell hardening. An atmosphere can follow the motion of the dislocation only very slowly, as in very slow creep.

ATOM. The smallest particle of an element which can enter into chemical combination. All chemical compounds are formed of atoms, the difference between compounds being attributable to the nature, number, and arrangement of their constituent atoms. For current views of structure, see atomic structure.

AVOGADRO CONSTANT. The number of molecules contained in one mole or grammolecular weight of a substance. A number of values of the Avogadro number, which is usually denoted by N, have been found by various methods, generally lying within a range of 1% about the value 6.02 X 1023

B. (1) Breadth or width (b). (2) Element boron (B). (3) "Boils at" (b). (4) Second van der Waals constant (b). (5) Effective film thickness (B). (6) Brightness or luminance (B). (7) Wien displacement constant (b). (8) Volume modulus of elasticity (B). (9) Factor

in Richardson-Dushman equation (b). (10) Susceptance (B or b). (11) Second Couchy constant (B). (12) Magnetic flux density (B). (13) Band head (B).

BB, B+, B- Letter and symbols used to identify the high-voltage plate supply for vacuum tubes.

B BATTERY. The power source for the anode circuit of an electronic device which is battery-operated.

BACKGROUND. A general term for the totality of the effects that are always present in physical apparatus, and above which a phenomenon must show itself in order to be measured. Such unrelated effects include the unwanted counts or currents from cosmic rays in electrical apparatus for measurement of radioactivity, developable grains on photographic plates that are unrelated to the phenomena investigated, noise in acoustical apparatus, and many others.

BADGER RULE. An empirical relation between the force constants and vibrational frequencies of the electronic states of diatomic molecules of the form

k e (re−dij )3=C11

Where k e is the force constant, reis the internuclear distance, d ij is a constant that is different for each type of molecule (it is 0.68A if both atoms are in the first row of the periodic system, 1.25A if they are in the second, etc.) and C11 may be taken as the same for all molecules (= 1.86 if k e is in 100,000 dynes/cm and re and d ij in A). The Badger rule may readily be applied to polyatomic molecules.

BALANCE. (1) A condition of partial or complete equilibrium, or adjustment. (See balance, mechanical.) (2) A well-known instrument used in weighing. While any type of "scales" used for weighing may properly be called a balance, the term usually refers to the equal-arm balance familiar in every laboratory.

The dynamics of this instrument is relatively simple unless the pans are allowed to oscillate independently of the beam, a condition which should be carefully avoided while taking readings. The balance may then be treated as a gravity pendulum suspended from the central knife edge or pivot, the pans and their loads being regarded as concentrated at the end knife edges Any slight excess weight ∆ w on one pan causes a change in the equilibrium position. The "sensitiveness" of the balance is appropriately expressed as the change in the equilibrium pointer reading per unit excess weight. More convenient in practical work is the reciprocal of the sensitiveness, which may be called the "stability." (3) An electrical network is in a condition of balance when it is so adjusted that an emf in one branch produces no current in another branch.

BALLISTIC MEASUREMENT. Any measurement in which an impulse is applied to the measuring device and the subsequent motion of the device is determined as a measure of the impulse. (See ballistic pendulum and galvanometer, ballistic.)

BALLISTIC PENDULUM. An instrument used for measuring the horizontal velocity component of a projectile. In its usual form it consists of a simple pendulum of mass M, and of natural frequency f. A projectile of mass m, moving with a velocity V strikes the bob and is imbedded in it. The maximum excursion X of the bob is then measured. Assuming that M≫ m and that little damping is present, it may be shown by application of conservation laws that

V=2πfXMm

BALLISTICS. The science which treats of the motion of masses projected into space, especially as associated with the motion of projectiles from guns and cannon. The complete path of a projectile is comprised of three separate and distinct phases. The first occurs in the bore of the gun, and the study of projectile motion here is that of interior ballistics. Secondly, there is the study of the path taken by the projectile as it flies through space from the gun to the target. This is exterior ballistics. Then, thirdly, there is the study of the penetration and the penetrating power of a projectile, which, for want of a better term, might be called penetration ballistics.

BAR. A unit of pressure in the metric system equal to one million dynes per square centimeter. It is slightly less than one atmosphere. Unfortunately, in acoustics the bar was once used to denote a

pressure of one dyne per square centimeter. The commonly used unit today is the microbar, which is one dyne per square centimeter.

BARKHAUSEN EFFECT. A series of minute "jumps'' in the magnetization of iron or other ferromagnetic substance as the magnetizing force is continuously increased or decreased; discovered by H. Barkhausen in 1919. The effect may be observed by winding on the specimen, along with the magnetizing coil, a secondary coil connected to some sensitive detector of current fluctuations, such as an oscillograph or an audio amplifier. As the magnetizing current is steadily increased, the current in the secondary circuit, instead of being constant, exhibits a succession of small, sharp peaks or maxima, which the amplifier reveals by a faint clicking or snapping sound.

BARLOW RULE. The volumes of space occupied by the various atoms in a given molecule are approximately proportional to the valencies of the atoms; whenever an element exhibits more than one kind of valency the lowest value is generally selected.

BARN. A unit nuclear cross section of the magnitude of 10 -24 square centimeter per nucleus.

BARNETT EFFECT. In 1915, S. J. Barnett discovered that a relatively long iron cylinder, when rotated at high speed about its longitudinal axis, developed a slight magnetization, the value of which was proportional to the angular speed. He found the magnetization to be about 1.5 X 10-6 c.g.s. electromagnetic unit per revolution per sec for a cylinder about 7 cm in diameter and 50 cm long. The effect was attributed to the influence of the impressed rotation upon the revolving electronic systems within the atoms. An

inverse effect was discovered about the same time by Einstein and de Haas; viz., an iron cylinder, suspended vertically, was observed to rotate slightly when suddenly magnetized.

BAROMETER. An instrument for measuring the pressure of the atmosphere

BAROSCOPE. An indicating pressure gauge, consisting of a U-tube partly filled with liquid, having one end open to the atmosphere, and the other end connected to a system whose pressure is to be observed. Also called an open-end manometer.

BASS. Frequencies at the lower end of the audible range.

BATTERY. Any group of duplicate units which are contributing individually to a common effect.By far the most common usage of the word is in reference to a collection of chemical cells for the production or storage of electrical energy. As such, the battery may be of the primary type, of which the individual unit is the primary cell, or it may be the ordinary storage battery.

BEAM. (1) A flow of electromagnetic radiation or of particles that is essentially unidirectional. (2) A straight or initially curved member which supports bearing loads without the aid of arch action (see specific types of beams which follow). (3) The term beam is also used to designate certain rolled-steel sections, such as the I-beam, which may be used either as beams, as defined in (2) above, or as columns.

BEAT(S). A series of alternate maxima and minima in vibration amplitude, produced by the interference of two wave trains of

different frequency. A familiar example arises in the case of musical sounds.

BEATING. A phenomenon in which two or more periodic quantities of different frequencies produce a resultant having pulsations of amplitude.

BEER LAW (1852). If two solutions of the same salt be made in the same solvent, one of which is, for example, twice the concentration of the other, the absorption due to a given thickness of the first solution should be equal to that of twice the thickness of the second. Sometimes written

I=I 0e−acx

where I is the intensity of light transmitted, Io is the intensity of the incident light, e is the natural logarithmic base, c is the concentration of the solution in moles per liter, x is the thickness of the transmitting layer, and a is the molar absorption coefficient. In general Beer's law is used for light of a particular narrow wavelength band and then α is the specular molar absorption coefficient. The Beer law is also written

I=I o10ecx

where e is known as the molar extinction coefficient. Not all solutions obey the Beer law. (See also the Bouguer law,)

BEL. A dimensionless unit for expressing the ratio of two values of power, the number of bels being the logarithm to the base 10 of the

power ratio. With PI and P2 designating two amounts of power and N the number of bels corresponding to the ratio Pi/Pa,

N=log10 ( P 1/ P2 )

BERNOULLI EQUATION. A first order non-linear differential equation

dydx

+ f ( x ) y=g ( x ) yn

It may be made linear by the substitution y=un−1, giving

dudx

+ (1−n ) f ( x ) u=(1−n ) g ( x )

BERNOULLI LAW (OR THEOREM). A statement of the law of the conservation of energy for steady flow of an inviscid fluid. If the

fluid may be regarded as incompressible, the sum,ρ+ 12

ρ v2+ρgh

(where p is local hydrostatic pressure,12

ρ v2is the kinetic pressure,

and gh is the local gravitational potential), is constant along any one streamline. If the flow is also irrotational, the sum is constant over the whole flow. The law may be generalized for compressible flow.

BETA. (1) Coefficient of volume expansion (β). (2) Plane angle (β). (3) Relativity ratio (β). (4) See also beta-particle and beta-ray. (5) Ratio of speed to speed of light (β). (6) The short-circuit current gain in a grounded-emitter, transistor amplifier (β=α / (1−α )), where α is the current amplification) .

BETA DISINTEGRATION. A radioactive transformation of a nuclide wherein the atomic number is changed by +1 or -1, and the mass number is unchanged. When the atomic number is increased by 1, negative β-particle emission occurs, and when the atomic number is decreased by 1, there is positive β-particle (positron) emission or electron capture.

BETA RAY. A stream of p-particles.

BINARY CELL. In computer work, an information- storing element which can have one or the other of two stable states.

BINOMIAL. A polynomial containing two terms.

BINOMIAL THEOREM. A rule for expanding (x + y)n , where n is a positive integer. The result is

( x+ y )n=xn+n xn−1 y+n (n−1 )

2 !nn−2 y2+…+ yn

The (k+1) th term is

(nk) xn−k y k

Where (nk) is the binominal coefficient.

BIOT-SAVART LAW. A law expressing the intensity of the magnetic field in the neighborhood of a long, straight wire carrying a steady current. If a permanent magnet is rigidly attached in any position to a rod or frame which is capable of rotation about such a

wire as an axis, it is found that there is no resultant torque about the wire. From this it ie readily shown that the field intensity varies inversely as the distance from the wire (see vector potential). If the current is i (abamperes) and the distance r (cm), the intensity is given by the Biot-Savart law as H = 2t/r (oersteds). The Ampere law is sometimes called by this name, since either of the two laws may be deduced from the other.