Where does permeability collide?

The measure of how strongly a material becomes magnetised in response to an applied magnetic field: it is the ratio of the magnetic flux density B to the magnetising field H, and it is often quoted as a relative permeability against the value for vacuum. Griffiths' Introduction to electrodynamics sets out how this permeability enters the constitutive relation of a magnetic medium and separates diamagnetic, paramagnetic, and ferromagnetic behaviour. The referent is an electromagnetic property of matter, expressed in henries per metre, and it has nothing to do with fluid passing through rock, molecules crossing a membrane, or influence crossing an organisational boundary.[1]

Unit

The ratio of flux density to magnetising field, in henries per metre

Recovery means

Not applicable

Measured by

Determined from magnetisation curves or inductance measurements, and reported as a relative permeability against vacuum

The capacity of a porous medium such as rock or soil to transmit a fluid under a pressure gradient: by Darcy's law the volumetric flow is proportional to this property and to the gradient, divided by the fluid's viscosity. Intrinsic permeability, measured in darcies or in square metres, is a property of the pore geometry alone, distinct from the particular fluid it carries. Neuzil's 1994 How permeable are clays and shales? compiled field and laboratory values spanning many orders of magnitude and examined how they are inferred. The referent is a transport property of the ground, not the magnetic response of a material, the leakiness of a cell membrane, or the openness of a social boundary.[2]

Unit

A medium's capacity to transmit fluid, in darcies or square metres

Recovery means

Not applicable

Measured by

Inferred from flow tests under a known gradient using Darcy's law, or from laboratory permeameter measurements on cores

The ease with which a biological membrane allows a given molecule or ion to cross it, summarised by a permeability coefficient that relates the flux across the membrane to the concentration difference driving it. Al-Awqati's 1999 One hundred years of membrane permeability revisited Overton's rule linking permeation to lipid solubility, and modern work extends it to protein channels and transporters, as in the selective permeability of the blood-brain barrier reviewed by Abbott and colleagues. The referent is a physiological transport property of a lipid bilayer, expressed in centimetres per second, and it is unrelated to a magnetic constant, the flow of water through rock, or a boundary between social roles.[3][4]

Unit

A membrane's ease of passage for a solute, as a coefficient in centimetres per second

Recovery means

Not applicable

Measured by

Estimated from tracer flux across the membrane relative to the concentration gradient, in cell or barrier assays

The degree to which a boundary around a social role, group, or organisation lets people, information, or influence pass across it: a highly permeable boundary lets one domain enter another, while an impermeable one keeps them apart. Ashforth, Kreiner, and Fugate's 2000 All in a day's work treats permeability, alongside flexibility, as a defining property of role boundaries that shapes how readily a person moves between roles such as work and home. The referent is a property of a socially constructed boundary, judged by the flows it permits, and it is not a magnetic constant, the conductivity of rock, or the leakiness of a cell membrane.[5]

Unit

How far a role or group boundary admits outside flows, as a graded judgement

Recovery means

Not applicable

Measured by

Assessed with survey scales and interviews about how far one role's demands, people, or information cross into another

In urban design, permeability refers to the number of alternative routes through an urban environment, which shapes how freely people can move. This meaning comes from Bentley (1985) in Responsive environments.[6]

Unit

Recovery means

Measured by