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 stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...

, 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 elect ...

. It was invented by

William Thomson, 1st Baron Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy at the University of Glasgow for 53 years, he did important ...

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

. The

magnetic force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...

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 magnitude 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, called the

Kibble balance A Kibble 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 metrological instrument that can realize ...

, that removes this source of inaccuracy by a calibration step, was 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 units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fran ...

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

kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...

, the world's standard of

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

. In this application, the Kibble balance functions in the reverse sense to the Ampere balance: it is used to weigh the International Prototype of the Kilogram, defining the kilogram in terms of an electric current and a voltage. Since voltage and current themselves are already defined in terms of fundamental physical constants like Planck's constant and the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...

, this has created a new definition of the kilogram in terms of these fundamental constants. This is a more stable definition than the previous one, which was based on the physical prototype kilogram which is vulnerable to damage and deterioration over the years.

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 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 ; in other words, a number whose ''square'' (the result of multiplying the number by itself, or  ⋅ ) is . For example, 4 and −4 are square roots of 16, because . E ...

of the

couple Couple or couples may refer to : Basic meaning *Couple (app), a mobile app which provides a mobile messaging service for two people *Couple (mechanics), a system of forces with a resultant moment but no resultant force *Couple (relationship), tw ...

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 centi-ampere balance ranges from 1 to 100 centi-amperes, the deci-ampere balance from 1 to 100 deci-amperes, the ampere balance from 1 to 100 amperes, etc.

References

* {{EB1911, wstitle=Amperemeter Electricity Measuring instruments