Appendix: A list of named units Amagat A unit of volume often used in the study of the equation of state of gases. By definition the molar volume of a gas at ace and 1 atmos­ phere is 1 amagat. The exact value of the unit depends upon the gas considered, but is approximately 22·4 lit res mole -1. There is also an amagat unit of density equal to the density of a gas in which 1 mole occupies a volume of 1 amagat. Ampere (A) The ampere is the practical unit of electric current, the absolute unit in the MKS system, and one of the seven basic SI units. An absolute ampere is the steady current which, when maintained in two parallel rectilinear conductors of infinite length and negligible cross-section and separated by a distance of 1 metre in free space, produces between these conductors a force of 2 x 10 -7 newton per metre length. There is also an international ampere which is defined electro­ chemically, being the current which under specified conditions will deposit 1·118 x 10 - 6 kilogram of silver from a silver nitrate coulometer. Ampere-hour (A h) The ampere-hour is a unit of quantity of electricity. It is the amount of electricity which is passed when 1 ampere has been flowing for 1 hour, i.e. 1 A h = 3600 coulombs. Ampere, Thermal The SI unit of thermal current. 1 thermal ampere corresponds to an entropy flow of 1 watt per kelvin. Formerly, the thermal ampere corresponded to a heat flow rate of 1 watt. Ampere-turn (A, sometimes At, AT) The ampere-turn is the practical, MKS, and SI unit for magneto­ motive force. 1 ampere-turn is the magnetomotive force resulting 321 Angstrom (1) from the passage of a current of 1 ampere through one turn of a coil. Angstrom (A) A unit of length equal to 10- 10 m. Avogadro's Number The number of individual atoms in a gram-atom, of ions in a gram-ion, or of molecules in a gram-molecule, is called Avogadro's number. The accepted value is 6·02252 x 1023 per mole. Balmer A term sometimes used for the unit of wave-number. It is the number of waves in a centimetre and has units of cm - 1. See also Kayser and Rydberg. Baud A pulse per second or alternatively one binary digit. (Named after J. M. E. Baudot.) Bel (B) A number used to express logarithmically the ratio of two powers. It is named after Alexander Bell and is defined as N = log (Pz/Ptl where PI and P z are the two powers. The practical unit is the decibel (dB) where 1 decibel = 0·1 bel. Biot (Bi) A c.g.s. unit of current. 1 biot is the constant current which, if maintained in two parallel rectilinear conductors of infinite length and negligible cross-section and with separation of 1 centimetre in a vacuum, would produce a force between the conductors equal to 2 dynes per centimetre of length. 1 biot = 10 amperes. Blondel A metric unit ofluminance. One blonde! is the luminance of a uniform diffuser emitting 1 lumen per square metre. 1 1 blondel = ~ candela metre - 1 • n 322 Coulomb (C) Bohr Magneton (PB) A unit of magnetic moment. eh PB = -4~ = 9·2741 x 10- 24 joule tesla- I . nm In c.g.s. units, PB equals 9· 2741 x 10 - 21 erg gauss -I. Bohr Radius (a o) The radius of the first Bohr orbit of the hydrogen atom. In SI, ao = h2 /npoc 2 me2 = 5·29177 x 10- 11 m. Boltzmann's Constant (k) k = 1.38041 x 1O- 23 joule deg- I = 8.6167 x 1O- 5 eV deg- I • Brewster (B) The unit used to measure the stress-optical constant when a material shows birefringence under an applied stress. A brewster is the number of angstroms per millimetre path by which one component of the light is retarded relative to the other if a stress of 1 bar (atmosphere) is applied in a direction perpendicular to the path of the light. 1 B = 10 - 12 Pa - I. Brig A ratio of two units to base 10 (named after Briggs). Clausius 1 clausius is the entropy associated with a temperature of 1 kelvin in which there is an increase in heat of 1000 calories. 1 clausius = 4186·8 joules kelvin - I. Compton Wavelength (h) I.e = ~ = 2·42631 X 10- 12 m (for electron). mc See also Compton Effect. Coulomb (C) The coulomb is the practical unit of electric charge and the absolute, 323 Coulomb, Thermal MKS and SI unit. The coulomb is the charge crossing the section of a conductor in which a steady current of 1 ampere flows for 1 second. As the coulomb is directly related to the charge on the electron, which is 1·602 x 10 - 19 coulomb, it may be considered as a fundamental electrical unit. In the c.g.s. electrostatic system, the statcoulomb is defined in terms of the electrostatic force between charges. A statcoulomb is that charge which, when placed 1 centimetre away from a like charge, repels it with a force of 1 dyne. 1 coulomb = 2·9959 x 109 statcoulombs. Coulomb, Thermal The SI unit of thermal charge. 1 thermal coulomb corresponds to an increase in entropy of 1 joule per kelvin. 1 thermal coulomb = 1 J K -1. Formerly, the unit corresponded to an amount of heat of 1 joule. Curie (Ci, formerly c) That quantity of any radioactive substance which has a decay rate of J 7 x 10 10 disintegrations per second. Dalton The name occasionally used for atomic mass number, equal to 1/12 the mass of a neutral carbon-12 atom. 1 dalton = 1·66033 x 10- 27 kg. Debye (D) A unit of electric dipole moment used for the dipole moments of molecules. 1 debye = 3· 33564 x 10 - 30 coulomb metre. Einstein A unit used to describe the photoenergy involved in a gram-molecule of a substance during a photochemical reaction. The photoenergy E in Einstein units is given by N ohv where No is Avogadro's number and v is the frequency of the electromagnetic radiation in hertz. Eotvos (E) The change In gravitational acceleration of 10 - 9 galileo over a 324 Fraunhofer ( F) horizontal distance of 1 centimetre. 1 E = 10- 9 m s-z (horizontal m)-I. Farad (F) 1 farad is the capacitance of a condenser having a charge of 1 coulomb when the potential difference across the plates is 1 volt. The energy stored is 0·5 joule. The unit is very large and the everyday unit is the microfarad, which equals 10- 6 F. Faraday The faraday is the electric charge carried by 1 gram-equivalent of an ion and is equal to 96487 coulombs. (This value uses the international scale of atomic masses.) Farad, Thermal The SI unit of thermal capacitance. 1 thermal farad is the thermal capacity for which an amount of entropy of 1 joule per kelvin added to a body raises its temperature by 1 kelvin. 1 thermal farad = 1 J K -z. Formerly, the thermal farad corresponded to 1 joule of heat resulting in a temperature rise of 1 kelvin. 1 thermal farad = 1 J K - I. Fermi An obsolete unit oflength equal to 10- 15 m. Radii of nuclei are of this order of magnitude. Finsen Unit (FU) A unit of intensity of ultraviolet light at a specified wavelength, usually 296·7 nm. 1 FU = 105 Wm- z. Fourier See Ohm Thermal Franklin (Fr) An electrostatic c.g.s. unit of charge. 1 franklin is that charge which exerts on an equal charge at a distance of 1 centimetre in a vacuum a force of 1 dyne. 1 Fr = (0, lie) coulomb. Fraunhofer (F) A unit of reduced width of a spectrum line given by W = 106 ll)..j). 325 Fresnel where W is the reduced width in fraunhofers, ). is the wavelength and L1;. is the equivalent width of the line. Fresnel A unit of optical frequency sometimes used in spectroscopy. 1 fresnel = 10 12 Hz. Gameo or Gal (Gal) Unit of acceleration in the C.g.s. system of units equal to 1 cm s - 2. Gauss (G) This unit is the c.g.s. unit of magnetic flux. If a straight wire is passed through a magnetic field so as to cut it at the rate of 1 cm s - 1 perpendicular to the direction of the induction, then the value of induction necessary to produce an electromotive force of 1 abvolt per centimetre length of the wire is 1 gauss. It is equal to 1 maxwell per square centimetre. The corresponding SI unit is the tesla: 1 tesla = 104 gauss. Gibbs A unit of adsorption. 1 gibbs is an adsorption of 10- 6 mole over 1 square metre. Gilbert (Gb) The c.g.s. electromagnetic unit of magnetomotive force. 1 gilbert is the magneto motive force resulting from the passage of a current of 411: abamperes through one turn of a coil. 1 gilbert = 10/411: ampere-turns. Hartree A unit of energy used in atomic studies and equal to me 4 /h 2 . 1 hartree ~ 4·8505 x 10- 18 J. See also Hartree System of Units. Hefnerkerze (HK) (Hefner) The unit of luminous intensity used in Germany until 1940. 1 Hefnerkerze ~ 0·901 international standard candle. 326 Kelvin (K, formerly OK) Helmholtz A metric unit of dipole moment per unit area. 1 helmholtz = 3· 33564 X 10 10 C m - 1.