The kilogram (symbol: kg) is the SI base unit of mass. A gram is defined as one thousandth of a kilogram. Conversion of units describes equivalent units of mass in other systems.
SI prefixes are used to name multiples and subdivisions of the kilogram. The most commonly used ones are:
tonne = 1 000 kilograms (strictly speaking, this should be named megagram, but the name is rarely used) (not to be confused with non-metric ton units)
gram = 1/1 000 kilogram
milligram = 1 thousandth of a gram = 1 millionth of a kilogram
microgram = 1 millionth of a gram = 1/(10^9) kilogram
The kilogram is the only one of the SI units which is still defined in relation to an artifact rather than to fundamental physical properties. It is also the only base unit that employs one of the prefixes.
The kilogram was originally defined as the mass of one litre of pure water at a temperature of 4 degrees Celsius and standard atmospheric pressure. This definition was hard to realize accurately, partially because the density of water depends ever-so-slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition of the kilogram.
To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter, and kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype ("Le Grand Kilo") roughly every 10 years. The international prototype kilogram was made in the 1880s.
By definition, the error in the repeatability of the current definition is exactly zero; however, in the usual sense of the word, it can be regarded as of the order of 2 micrograms. This is found by comparing the official standard with its official copies, which are made of roughly the same materials and kept under the same conditions. There is no reason to believe that the official standard is any more or less stable than its official copies, thus giving a way to estimate its stability. This procedure is performed roughly once every forty years.
The international prototype of the kilogram seems to have lost about 50 micrograms in the last 100 years, and the reason for the loss is still unknown (reported in Der Spiegel, 2003 #26). The observed variation in the prototype has intensified the search for a new definition of the kilogram. It is accurate to state that any object in the universe (other than the reference metal in France) that had a mass of 1 kilogram 100 years ago, and has not changed since then, now is considered to have a mass which is 50 micrograms larger than a kilogram. This perspective is counterintuitive and defeats the purpose of a standard unit of mass, since the standard should not change arbitrarily over time.
Proposed future definitions
There is an ongoing effort to introduce a new definition for the kilogram by way of fundamental or atomic constants. The proposals being worked on are:
- The Avogadro approach attempts at defining the kilogram by a fixed count of silicon atoms. As a practical realization, a sphere will be used where the size is measured by interferometry.
- The ion accumulation approach involves accumulation of gold atoms and measuring the electrical current required to neutralise them.
- The Watt balance uses the current balance that formerly was used to define the ampere to relate the kilogram to a value for Planck's constant, based on the definitions of the volt and the ohm.
- The levitated superconductor approach relates the kilogram to electrical quantities by levitating a superconducting body in a magnetic field generated by a superconducting coil, and measuring the electrical current required in the coil.
- Since the values of the Josephson (CIPM (1988) Recommendation 1, PV 56; 19) and von Klitzing (CIPM (1988), Recommendation 2, PV 56; 20) constants have been given conventional values, it is possible to combine these values (KJ ≡ 4.835 979×1014 Hz/V and RK ≡ 2.581 280 7×104 Ω) with the definition of the ampere to define the kilogram. As follows:
- The kilogram is the mass which would be accelerated at precisely 2×10-7 m/s² if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6.241 509 629 152 65 × 1018 elementary charges per second.
Link with weight
When a property of an object is given in kilograms, the property intended is almost always mass, but the property in common usage is often called its "weight", a usage much deprecated by those communities (physicists and engineers) that prefer weight always to mean "gravitational force". Occasionally the gravitational force on an object is given in "kilograms", but the unit used is not a true kilogram: it is the deprecated kilogram-force (kgf), also known as the kilopond (kp). An object of mass 1 kg at the surface of the Earth will be subjected to a gravitational force (that is to say, it will have a weight) of approximately:
1 kgf = 1 kg × 9.806 65 m/s² = 9.806 65 N
where N represents the newton, the SI unit of force. Note that the factor of 9.806 65 is only an agreed-upon average (3rd CGPM (1901), CR 70), as the exact value of g, the local gravitational acceleration, varies with altitude and location on the Earth. (See standard gravity).