| 释义 |
permeability|ˌpɜːmiːəˈbɪlɪtɪ|
[f. permeable + -ity: in F. perméabilité (1625 in Hatz.-Darm.).]
1. a. The quality or condition of being permeable; capability of being permeated; perviousness. Also, the degree to which a solid allows the passage of fluid through it, measured by the coefficient of permeability (or permeability coefficient), the volume of fluid flowing through unit cross-section in unit time under a unit (pressure or concentration gradient).
1759Wilson in Phil. Trans. LI. 328 Confirmations of the permeability of glass. 1805W. Saunders Min Waters 487 The permeability of the skin to heat. 1882Geikie Text-bk. Geol. iii. ii. ii. §2. 351 The permeability of subterranean rocks. 1902Sci. Abstr. V. 856 (heading) Permeability of animal membranes. Ibid. 857 Inactive membranes..offer but little resistance to the passage of the different ions, and the permeability does not alter by contact with the solution. 1917Rep. & Mem. Advisory Comm. Aeronaut. No. 317. 4 This permeameter is readily adapted to the determination of temperature coefficients of permeability. 1920Ibid. No. 360. 4 The average permeability of rubbered airship fabrics..is not usually much less than 10 litres per sq. metre per day. 1931Jrnl. Gen. Physiol. XIV. 408 This definition of permeability is seen to possess a definite physical meaning viz., the number of cubic micra of water entering the cell per minute per unit area of membrane, per atmosphere of difference in osmotic pressure between interior and external medium. 1960Ibid. XLIII. 523 (heading) Experimental study of the independence of diffusion and hydrodynamic permeability coefficients in collodion membranes. 1962R. C. S. Walters Dam Geol. xiv. 67 The permeability [of the Oxford clay] ranged from 12 × 10-6 to 4·1 × 10-6 cm per sec. 1966McGraw-Hill Encycl. Sci. & Technol. XII. 453/2 The permeability coefficient varies from 100 cm/sec for clean gravel to 10-9 cm/sec for heavy clay.
b. magnetic (etc.) permeability: see quot. 1872. (magnetic) permeability, also, one of the physical parameters of a medium, equal to the ratio of the magnetic induction B to the magnetic field strength H at any point in it; also (more fully relative permeability), the ratio of the permeability of a medium to the permeability of free space; permeability of free space, a constant µ0 which in the C.G.S. electromagnetic system of units is unity and in the International System of Units is defined as a base quantity with the value 4π (12·57) × 10-7 henry per metre.
Of the different permeabilities mentioned in quot. 1872, the magnetic kind is the only one that has gained currency.
1872Thomson in Papers Electrostatics & Magn. 484 We have thermal permeability, a synonym for thermal conductivity; permeability for lines of electric force, a synonym for the electro-static inductive capacity of an insulator; magnetic permeability, a synonym for conducting power for lines of magnetic force. 1892J. A. Ewing Magn. Induction ii. 56 Prof. Knott has proposed to call this quantity the ‘differential susceptibility’; similarly dB/dH may be called the differential permeability. 1896Bedell Princ. of Transf. 40 The reluctance, R, or magnetic resistance..varies..inversely as the cross-section and permeability. 1939L. F. Bates Mod. Man. ii. 67 Pure iron has an initial permeability of about 250. 1942Phil. Mag. XXXIII. 488 The universal constant 1/ac2 occurring in (12.2) has the value..1/107 ohm sec./m...and is called the permeability of empty space. Consistency practically compels us to call the constant µ/ac2 in (12.3) the permeability of the medium. 1944[see permittivity]. 1962Corson & Lorrain Introd. Electromagn. Fields v. 177 The constant µ0 is called the permeability of free space and is arbitrarily taken to be exactly 4π × 10-7 newton/ampere2 in rationalized m.k.s. units. Ibid. vii. 284 Figure 7–17 shows magnetization data for several different kinds of iron, with lines of constant relative permeability indicated. The detailed shape of the magnetization curve and the maximum permeability achieved with a given sample of iron depend on the purity, the method of annealing, and on the thickness of the sheets. 1971Nature 16 July 208/2 The indiscriminate use of the symbol µ..both for the permeability of free space and that of ferromagnetic materials may temporarily confuse the unwary.
2. Special Comb.: permeability tuning Electronics, tuning in which the resonant frequency of a circuit is changed by moving a magnetic core into or out of a coil forming part of it, so as to change its inductance.
1933W. J. Polydoroff in Proc. IRE XXI. 694 The apparent inductance is increased to tune to lower frequencies by introducing a magnetic core into the field of the coil. As the core is inserted into the coil, more lines of the magnetic field are intercepted by the core, and, in effect, the average apparent permeability of the medium surrounding the coil increases from 1 (for air) to a certain maximum... For this reason, and for other reasons.., it is appropriate to describe this method as ‘Permeability Tuning’. 1968Radio Communication Handbk. (ed. 4) iv. 35/2 Although permeability tuning has been most successfully used for many years by one major American company, the mechanical and electrical complications involved in band-switched receivers have resulted in little progress in this field by amateur constructors. |