Where does couple collide?

Two forces equal in magnitude and opposite in direction whose lines of action do not coincide, so their net force is zero but together they exert a pure turning effect on a body. Hibbeler's 2016 Engineering mechanics: Statics defines a couple as a pair of such forces separated by a perpendicular distance, and shows that the moment they produce, the couple moment, is the same about every point and depends only on the force and that separation. The referent is a mechanical action that rotates without translating, measured as a moment in newton metres, and it is not a pair of chemical species, a temperature sensor, or two people in a partnership.[1]

Unit

A pure turning effect from two equal, opposite, offset forces, as a moment in newton metres

Recovery means

Not applicable

Measured by

Computed as the product of one force and the perpendicular distance between the two lines of action, giving the couple moment

A matched pair of chemical species, one oxidised and one reduced, that are interconverted by the gain or loss of electrons and together define a half reaction and its electrode potential. Bard and Faulkner's 2001 Electrochemical methods: Fundamentals and applications treats a redox couple such as Fe(III)/Fe(II) as the two forms whose ratio sets the potential of an electrode through the Nernst equation. The referent is a conjugate pair of oxidised and reduced forms, described by a standard potential in volts, and it has nothing in common with two offset forces, a joined pair of metals, or a human partnership beyond the word.[2]

Unit

A conjugate oxidised and reduced pair defining a half reaction, by its standard electrode potential in volts

Recovery means

The pair is restored when the reverse electron transfer returns each species to its original oxidation state

Measured by

Determined from the electrode potential of the half cell, related to the ratio of oxidised to reduced forms by the Nernst equation

A temperature sensor made of two wires of dissimilar conductors joined at one end, which develops a small voltage that depends on the difference in temperature between that junction and the reference end. Kerlin and Johnson's 2012 Practical thermocouple thermometry explains how this thermoelectric, or Seebeck, effect lets the measured voltage be read back as a temperature. The referent is a physical measuring device, a joined pair of metals calibrated in degrees, and although it too is a couple of two things, it is not a pair of forces, a pair of chemical species, or a pair of people.[3]

Unit

A joined pair of dissimilar conductors sensing temperature, read as a thermoelectric voltage in millivolts

Recovery means

Not applicable

Measured by

The voltage across the two wires is measured and converted to temperature using the metal pair's calibrated characteristic