Femtoliter

The litre (international spelling) or liter (American spelling) (symbols L, l or ℓ) is an SI accepted metric system unit of volume equal to 1 cubic decimetre (dm3), 1,000 cubic centimetres (cm3) or 1/1,000 cubic metre. A cubic decimetre (or litre) occupies a volume of 10 cm×10 cm×10 cm (see figure) and is thus equal to one-thousandth of a cubic metre. The original French metric system used the litre as a base unit. The word litre is derived from an older French unit, the litron, whose name came from Greek — where it was a unit of weight, not volume — via Latin, and which equalled approximately 0.831 litres. The litre was also used in several subsequent versions of the metric system and is accepted for use with the SI, although not an SI unit — the SI unit of volume is the cubic metre (m3). The spelling used by the International Bureau of Weights and Measures is 'litre', a spelling which is shared by almost all English-speaking countries. The spelling 'liter' is predominantly used in American English. One litre of liquid water has a mass of almost exactly one kilogram, because the kilogram was originally defined in 1795 as the mass of one cubic decimetre of water at the temperature of melting ice (0°C). Subsequent redefinitions of the metre and kilogram mean that this relationship is no longer exact. A litre is defined as a special name for a cubic decimetre or 10 centimetres × 10 centimetres × 10 centimetres, (1 L ≡ 1 dm3 ≡ 1000 cm3). Hence 1 L ≡ 0.001 m3 ≡ 1000 cm3, and 1 m3 (i.e. a cubic metre, which is the SI unit for volume) is exactly 1000 L. From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density (+4°C) and standard pressure. The kilogram was in turn specified as the mass of a platinum/iridium cylinder held at Sèvres in France and was intended to be of the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3. Additionally, the mass-volume relationship of water (as with any fluid) depends on temperature, pressure, purity and isotopic uniformity. In 1964, the definition relating the litre to mass was abandoned in favour of the current one. Although the litre is not an SI unit, it is accepted by the CGPM (the standards body that defines the SI) for use with the SI. CGPM defines the litre and its acceptable symbols. A litre is equal in volume to the millistere, an obsolete non-SI metric unit customarily used for dry measure. Litres are most commonly used for items (such as fluids and solids that can be poured), which are measured by the capacity or size of their container, whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water.