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Physical unit
Unit name amp
Unit symbol
Physical quantity (s) Amperage , magnetic flow
Formula symbol
system International system of units
In SI units Base unit
Named after André-Marie Ampère

The ampere [ amˈpɛɐ̯ ] with unit symbol  A, named after the French mathematician and physicist André-Marie Ampère , is the SI - base unit of electrical current strength and at the same time SI unit of the derived quantitymagnetic flux ”.

Although the surname of the namesake is written with accent grave ("Ampère"), the SI unit in the German and English-speaking countries is usually written without an accent, ie "Ampere".


The blue circles represent electrons flowing through the cross-sectional area of ​​the conductor. One ampere corresponds to one coulomb that flows through the conductor cross-section in one second.

One ampere corresponds to a flow of 1  coulomb (C) per second through the conductor cross-section:

In the International System of Units , the coulomb is defined by the specified elementary charge. One ampere therefore corresponds exactly to a flow of elementary charges per second, with a flow of electrons this is approx. 6.2 · 10 18 (6.2  trillion ) electrons per second.

A flow of 1 A over a voltage of 1  volt (V) means an output of 1  watt (W).


Before the ampere was established as the international unit of current, there were a number of different units and definitions. In Germany and some other countries, the " Weber unit" of the current was in use, 1 Weber unit = 0.1 ampere. In Great Britain it was initially suggested that the unit of current strength should be named “Galvat” after the Italian biophysicist Luigi Galvani , which would have roughly corresponded to today's ampere. Later a "Weber unit" was also introduced for the amperage, but it had a value ten times higher than the one commonly used in Germany. It became even more complicated because the name Weber was also used for the unit of electrical charge , so that the current intensity was then equal to "Weber unit / second". At times the Weber unit was also given the name "Farad", which was later used to name the unit of electrical capacitance .

Amperage as the base unit

If one were to measure the current intensity with a derived unit , as is the case with the CGS system of units , the electrical quantities could only be expressed by the base units with non-integer exponents. To avoid this, the unit of amperage was proposed as a further basic unit as early as 1939 .

Definition 1898

1898, 1 ampere in the "Law concerning the electrical units" of the German Empire defined as the strength of that current, which from an aqueous silver nitrate - solution by means of electrolysis in a second 1.118  mg silver deposits. The ampere so defined was later referred to as the international ampere ; the one compatible with the rest of the base units, on the other hand, as an absolute ampere .

Definition 1948

Illustration of the ampere definition from 1948. The forces drawn in blue each have the amount 2 · 10 −7 N.

In 1948 the ampere was defined as follows using the Lorentz force of two conductors on each other:

1 A is the strength of the temporally constant electric current , which in the vacuum between two parallel , infinitely long, straight conductors with a negligibly small, circular cross-section and the distance of 1 m between these conductors, a force of 2 · 10 −7 Newtons per meter of conductor length would evoke.

With this definition, the value of the magnetic field constant μ 0 was also determined.

Definition since 2019

On November 16, 2018, at the 26th General Conference on Weights and Measures , it was decided to redefine the ampere and other SI base units with effect from May 20, 2019. With this redefinition of the International System of Units , the ampere is based on the elementary charge to which a fixed numerical value has been assigned. Since then, the definition of the ampere depends only on the definition of the second , but no longer on the meter or kilogram . The redefinition was made because it is easier to implement.

Since then, the magnetic field constant μ 0 has been a measured variable with a measurement uncertainty that has to be determined experimentally.

Web links

Wiktionary: Ampere  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. a b New definitions in the International System of Units (SI). (PDF; 1.3 MB) PTB , accessed on October 30, 2019 .
  2. ^ JG Crowther: British Scientists of the Nineteenth Century , Routledge & Kegan Paul, London, 2009, p. 246.
  3. ^ Wilhelm Jaeger: The emergence of the international dimensions of electrical engineering , Julius Springer Verlag, Berlin 1932, pp. 8–9.
  4. Law on Wikisource .
  5. Ernst Schmidt: State of our knowledge of the basic units and constants of physics and technology . In: Natural Sciences . tape 34 , no. 3 , 1947, pp. 93 , doi : 10.1007 / BF00663124 .
  6. The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e edition, 2006)". In: PTB-Mitteilungen . tape 117 , no. 2 , 2007, p. 156 ( [PDF; 1.4 MB ]). PDF; 1.4 MB ( memento from March 4, 2016 in the Internet Archive ).
  7. a b 26th CGPM (2018) - Resolutions adopted / Résolutions adoptées. (PDF; 1.2 MB) Versailles 13–16 November 2018. In: Bureau International des Poids et Mesures, November 19, 2018, pp. 2–5 , accessed on May 6, 2019 (English, French).