Barye
The barye (symbol: Ba), or sometimes barad, is the centimetre–gram–second (CGS) unit of pressure.[1] It is equal to one dyne per square centimetre (1 Ba = 1 dyn/cm²) or 0.1 pascal (1 Ba = 0.1 Pa) in the International System of Units (SI).[2] The name derives from the Greek word βάρος (barys), meaning "heavy" or "weight".[3] Although rarely used today, it remains part of the CGS system for expressing pressure in scientific contexts.Physical Definition
Definition in CGS System
The barye (symbol: Ba) is the centimeter–gram–second (CGS) unit of pressure, defined as the pressure exerted when a force of one dyne acts uniformly over an area of one square centimeter.[4] This unit forms part of the absolute CGS system, which derives coherent units from the base quantities of length (centimeter), mass (gram), and time (second), such as the dyne for force and the erg for energy; it measures pressure in absolute terms, without reference to atmospheric conditions, distinguishing it from any gauge pressure variants.[5] The general relation for pressure in the CGS system is given by P = \frac{F}{A}, where P is pressure in baryes, F is force in dynes, and A is area in square centimeters, such that $1 Ba = 1 dyn/cm².[4] Historically, the unit was also known by the alternative name barad, proposed by a committee of the British Association in 1888 as an absolute unit of pressure equivalent to one dyne per square centimeter; this and other variant names are now obsolete in favor of the standardized term barye.[6][7]Relation to Force and Area
In the centimetre-gram-second (CGS) system of units, pressure is conceptually defined as the force applied per unit area, ensuring that all derived quantities maintain dimensional coherence with the base units of mass, length, and time. The barye (symbol: Ba), the coherent CGS unit of pressure, is directly derived from the system's fundamental units of force and area. Specifically, one barye equals one dyne per square centimetre, where the dyne represents the force and the square centimetre the area.[8][2] The dyne, as the CGS unit of force, is defined as the force required to accelerate a mass of one gram at a rate of one centimetre per second squared. With area measured in square centimetres, the barye thus embodies the expression for pressure as P = \frac{F}{A}, where F is force in dynes and A is area in square centimetres, yielding units of dyn/cm². This derivation upholds the CGS framework's consistency, as pressure's dimensions—[M][L]^{-1}[T]^{-2}—align precisely with those of the base units (gram for mass, centimetre for length, second for time). For example, applying a force of 1 dyne uniformly over an area of 1 cm² produces a pressure of exactly 1 Ba, illustrating the system's uniform scaling in mechanical calculations.[9][10][11] The barye specifically quantifies absolute pressure in the CGS system, measuring the total force per unit area without reference to external conditions like atmospheric pressure. This distinguishes it from non-coherent units such as the technical atmosphere, which employs kilogram-force per square centimetre and relates to standard atmospheric values rather than pure CGS primitives.[12]Numerical Equivalents
Value in SI Units
The barye (Ba), the centimeter-gram-second (CGS) unit of pressure, is equivalent to 0.1 pascals (Pa) in the International System of Units (SI), where the pascal is defined as one newton per square meter (N/m²).[13] This equivalence arises from the definitions of base units in the CGS and SI systems. In the CGS system, 1 Ba equals 1 dyne per square centimeter (dyn/cm²), where the dyne is the unit of force and equals $10^{-5} newtons (N).[14] One square centimeter (cm²) corresponds to $10^{-4} square meters (m²), as 1 cm = 0.01 m.[14] Thus, the pressure conversion is derived as follows: $1 \, \text{Ba} = \frac{1 \, \text{dyn}}{1 \, \text{cm}^2} = \frac{10^{-5} \, \text{N}}{10^{-4} \, \text{m}^2} = 10^{-1} \, \text{Pa} = 0.1 \, \text{Pa}. [13][14] Conversely, 1 Pa equals 10 Ba.[13] Given its small scale—0.1 Pa represents a very low pressure—the barye is particularly suited for quantifying micro-pressures in phenomena such as acoustics and vacuum conditions.[13]Comparison to Other Pressure Units
The barye (Ba), as the CGS unit of pressure, equates to 1 microbar (μbar), making it directly interchangeable with this subunit of the bar in certain contexts.[3] Conversely, larger units scale up significantly: 1 bar equals 10⁶ Ba, reflecting the bar's definition as 10⁵ Pa while 1 Ba is 0.1 Pa.[15][16] Standard atmospheric pressure stands at approximately 1.01325 × 10⁶ Ba, 1 pound per square inch (psi) at about 6.89476 × 10⁴ Ba, and 1 torr at roughly 1333.22 Ba.[15][16]| Unit | Symbol | 1 Unit = Ba (barye) | Typical Use |
|---|---|---|---|
| Barye | Ba | 1 | Legacy CGS system in physics |
| Microbar | μbar | 1 | Sub-atmospheric pressures |
| Torr | Torr | ≈1333.22 | Vacuum technology and manometry |
| Pound per square inch | psi | ≈6.89476 × 10⁴ | Engineering and hydraulics in the US |
| Bar | bar | 10⁶ | Meteorology and high-pressure systems |
| Standard atmosphere | atm | ≈1.01325 × 10⁶ | Reference for ambient conditions |