Gauge

Gauge (sometimes spelled gage) is used with differing meanings in mechanics, engineering, mathematics and physics.

In mechanics, a pressure gauge is a device for indicating liquid or gas pressure, a vacuum gage indicates gas pressure less than atomospheric pressure.

In engineering, gauge refers to a measure of width or thickness, or to devices used to make measurements (originally of width or thickness). It can also be used as a verb to describe the act of taking a measurement, typically an estimate.

Gauge is commonly used in the context of railways, where 'track gauge' (often simply 'gauge') means the distance between the inside edges of the two rails forming the track (eg standard gauge, narrow gauge) and 'loading gauge' refers to the set of height and width profiles governing the maximum dimensions of railway vehicles.

The term is also used in the measurement of metal sheeting, where it refers to the thickness of the sheet.

In shotguns, gauge is related to the diameter of the barrel. The gauge is determined by the number of solid spheres of a diameter equal to the inside diameter of the barrel that could be made from a pound of lead.

It is more rarely used to refer to the internal dimensions of a cannon.

In mathematics and physics, a gauge transformation is a member of a group of mappings to a space or a spacetime, where this group of mappings satisfies certain properties. The actionss of bosons, which mediate interactions between fermions, in the theories of the electroweak interaction and quantum chromodynamics of the Standard Model of particle physics, are invariant under gauge transformations, so these bosons are called gauge bosons. Gauge theories in physics postulate that symmetry transformations can only be performed locally.