The ampere balance (also current balance or Kelvin balance) is an electromechanical apparatus used for the precise measurement of the SI unit of

electric current An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...

, the

ampere The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...

. It was invented by

William Thomson, 1st Baron Kelvin William Thomson, 1st Baron Kelvin (26 June 182417 December 1907), was a British mathematician, Mathematical physics, mathematical physicist and engineer. Born in Belfast, he was the Professor of Natural Philosophy (Glasgow), professor of Natur ...

. The current to be measured is passed in series through two coils of wire, one of which is attached to one arm of a sensitive

balance Balance may refer to: Common meanings * Balance (ability) in biomechanics * Balance (accounting) * Balance or weighing scale * Balance, as in equality (mathematics) or equilibrium Arts and entertainment Film * Balance (1983 film), ''Balance'' ( ...

. The

magnetic force Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...

between the two coils is measured by the amount of weight needed on the other arm of the balance to keep it in equilibrium. This is used to calculate the numerical value of the current. The main weakness of the ampere balance is that the calculation of the current involves the dimensions of the coils. So the accuracy of the current measurement is limited by the accuracy with which the coils can be measured, and their mechanical rigidity. A more complicated version of an ampere balance, that removes this source of inaccuracy by a calibration step, is the

Kibble balance A Kibble balance (also formerly known as a watt balance) is an electromechanical measuring instrument that measures the weight of a test object very precisely by the electric current and voltage needed to produce a compensating force. It is a Me ...

, invented by

Bryan Kibble Bryan Peter Kibble (20 October 1938 – 28 April 2016) was a British physicist and a pioneering metrologist. He was the inventor of the Kibble balance, an improved version of the current balance, developed for the realisation of the S.I. unit ...

in 1975. This experimental device was developed at government

metrology Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to stan ...

laboratories worldwide with the goal of providing a more accurate definition of the

kilogram The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...

, the world's standard of

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

. In this application, the Kibble balance functions in the reverse sense to the Ampere balance: it was used to weigh the

International Prototype of the Kilogram The International Prototype of the Kilogram (referred to by metrology, metrologists as the IPK or Le Grand K; sometimes called the ''wiktionary:ur-#Prefix, ur-kilogram'', or ''urkilogram'', particularly by German-language authors writing in Engli ...

, defining the kilogram in terms of an electric current and a voltage. In 2019, the kilogram, ampere, kelvin, and mole were redefined in terms of fundamental constants, removing the dependence on physical objects.

Usage

Approximate readings may be obtained by reading the position of the weight on the scale, or a more accurate reading may be obtained as follows: The upper edge of the shelf on which the weights slide is graduated into equal divisions, and the weight is provided with a sharp tongue of metal in order that its position on the shelf may be accurately determined. Since the current passing through the balance when

equilibrium Equilibrium may refer to: Film and television * ''Equilibrium'' (film), a 2002 science fiction film * '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film * "Equilibrium" (''seaQuest 2032'') * ''Equilibr ...

is obtained with a given weight is proportional to the

square root In mathematics, a square root of a number is a number such that y^2 = x; in other words, a number whose ''square'' (the result of multiplying the number by itself, or y \cdot y) is . For example, 4 and −4 are square roots of 16 because 4 ...

of the couple due to this weight, it follows that the current strength when equilibrium is obtained is proportional to the product of the square root of the weight used and the displacement of this weight from its zero position. Each instrument is accompanied by a pair of weights and by a square root table, so that the product of the square roots of the number corresponding to the position of the sliding weight and the ascertained constant for each weight, gives at once the value of the current in amperes. Each of these balances is made to cover a certain range of reading. Thus, the centiampere balance ranges from 1 to 100 centiamperes, the deciampere balance from 1 to 100 deciamperes, the ampere balance from 1 to 100 amperes, etc.

References

* {{EB1911, wstitle=Amperemeter Electricity Measuring instruments