Origin of SI units - Litre

The litre  (SI symbols L or l) is a non-SI metric system unit of volume equal to 1 cubic decimetre (dm³), 1000 cubic centimetres (cm³) or  1/1000 cubic metre. A cubic decimetre (or litre) occupies a volume of 10×10×10 centimetres 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 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 official 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.

One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice. Subsequent redefinitions of the metre and kilogram mean that this relationship is no longer exact by definition.

The first name of the litre was "cadil"; standards are shown at the Musée des Arts et Métiers in Paris. Then the litre was introduced in France in 1795 as one of the new 'republican units of measurement' and defined as one cubic decimetre. One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice. The original decimetre length was 44.344 lignes, which was revised in 1798 to 44.3296 lignes. This made the original litre 1.000974 of today's dm³. It was against this litre that the kilogram was constructed.

In 1879, the CIPM adopted the definition of the litre, with the symbol l (lowercase letter L).

In 1901, at the 3rd CGPM conference, the litre was redefined as the space occupied by 1 kg of pure water at the temperature of its maximum density (3.98 °C, 277.13 K) under a pressure of 1 atm. This made the litre equal to about 1.000028 dm³ (earlier reference works usually put it at 1.000027 dm³).

In 1964, at the 12th CGPM conference, the original definition was reverted to, and thus the litre was once again defined in exact relation to the metre, as another name for the cubic decimetre, that is, exactly 1 dm³.

In 1979, at the 16th CGPM conference, the alternative symbol L (uppercase letter L) was adopted. It also expressed a preference that in the future only one of these two symbols should be retained, but in 1990 said it was still too early to do so.

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.

One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 277.13 K (3.98 °C). Similarly: 1 millilitre of water has a mass of about 1 g; 1,000 litres of water has a mass of about 1000 kg (1 tonne). This relationship holds because the gram was originally defined as the mass of 1 mL of water; however, this definition was abandoned in 1799 because the density of water changes with temperature and, very slightly, with pressure.

Now 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.

The litre, though not an official SI unit, may be used with SI prefixes. The most commonly used derived unit is the millilitre, defined as one-thousandth of a litre, and also often referred to by the SI derived unit name 'cubic centimetre'. It is a commonly used measure, especially in medicine and cooking.

Originally, the only symbol for the litre was l (lowercase letter L), following the SI convention that only those unit symbols that abbreviate the name of a person start with a capital letter. In many English-speaking countries, however, the most common shape of a handwritten Arabic digit 1 is just a vertical stroke; that is, it lacks the upstroke added in many other cultures. Therefore, the digit '1' may easily be confused with the letter 'l'. Further, on some typewriters, particularly older ones, the unshifted L key had to be used to type the numeral 1. Even in some computer typefaces, the two characters are barely distinguishable. This caused some concern, especially in the medical community.

As a result, L (uppercase letter L) was adopted as an alternative symbol for litre in 1979. The United States National Institute of Standards and Technology now recommends the use of the uppercase letter L, a practice that is also widely followed in Canada and Australia. In these countries, the symbol L is also used with prefixes, as in mL and µL, instead of the traditional ml and µl used in Europe. In the UK and Ireland as well as the rest of Europe, lowercase l is used with prefixes, though whole litres are often written in full (so, '750 ml' on a wine bottle, but often '1 litre' on a juice carton). In 1990, the CIPM stated that it was still too early to choose a single symbol for the litre.

Prior to 1979, the symbol ℓ (script small l, U+2113), came into common use in some countries; for example, it was recommended by South African Bureau of Standards publication M33 and Canada in the 1970s. This symbol can still be encountered occasionally in some English-speaking countries, and its use is ubiquitous in Japan and South Korea. Fonts covering the CJK characters usually include not only the script small ℓ but also four precomposed characters: ㎕, ㎖, ㎗ and ㎘ (U+3395 to U+3398) for the microlitre, millilitre, decilitre and kilolitre. Such usage in printed works is in conflict with the recommendations published by the BIPM on the advice of the major international standards organisations (including ISO, NIST, NPL, IAU, IUPAC and IUPAP) who state in the SI Brochure that unit symbols should be 'printed in Roman (upright) type regardless of the type used in the surrounding text'.

In this post we introduced a metric unit accepted for using with SI - litre. In the next post we will talk about another metric unit accepted for using with SI - tonne.