How Many? A Dictionary of Units of Measurement
© Russ Rowlett and the University of North Carolina at Chapel Hill

Table of Contents
About the Dictionary
Using the Dictionary

Base Units of the International System (SI)

The General Conference on Weights and Measures has replaced all but one of the definitions of its base (fundamental) units based on physical objects (such as standard meter sticks or standard kilogram bars) with physical descriptions of the units based on stable properties of the Universe.

For example, the second, the base unit of time, is now defined as that period of time in which the waves of radiation emitted by cesium atoms, under specified conditions, display exactly 9 192 631 770 cycles. The meter, the base unit of distance, is defined by stating that the speed of light, a universal physical constant, is exactly 299 792 458 meters per second. These physical definitions allow scientists to reconstruct meter standards or standard clocks anywhere in the world, or even on other planets, without referring to a physical object kept in a vault somewhere.

In fact, the kilogram is the only base unit still defined by a physical object. The International Bureau of Weights and Measures (BIPM) keeps the world's standard kilogram in Paris, and all other standards of mass, such as those of Britain and the United States, are calibrated against this standard kilogram.

This one physical standard is still used because scientists can weigh objects very accurately. Mass standards in other countries can be adjusted to the Paris standard kilogram with an accuracy of one part per hundred million. So far, no one has figured out how to define the kilogram in any other way that can be reproduced with better accuracy than this. The 21st General Conference on Weights and Measures, meeting in October 1999, passed a resolution calling on national standards laboratories to press forward with research to "link the fundamental unit of mass to fundamental or atomic constants with a view to a future redefinition of the kilogram." The 22nd General Conference, in 2003, renewed this request. It is possible that the 24th General Conference, in 2007, will make a change in the definition.

Following are the official definitions of the seven base units, as given by BIPM. The links in the first column are to my (possibly) less obscure definitions.

meter (m)


"The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."

kilogram (kg)


"The kilogram is equal to the mass of the international prototype of the kilogram."

second (s)


"The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

ampere (A)

electric current

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length."

kelvin (K)


"The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."

mole (mol)

amount of substance

"The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."

candela (cd)

intensity of light

"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."



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All material in this folder is copyright © 2002 by Russ Rowlett and the University of North Carolina at Chapel Hill. Permission is granted for personal use and for use by individual teachers in conducting their own classes. All other rights reserved. You are welcome to make links to this page, but please do not copy the contents of any page in this folder to another site. The material at this site will be updated from time to time.

October 7, 2002