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SI Units – Electric Current

SI Units ampere banner
Credit: NIST
A - Ampere - Electric Current - 2018

The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634 × 10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆νCs.

The SI unit of electric potential difference is the volt (V) 1 V = 1 W/A.

The SI unit of electric resistance is the ohm (Ω). 1 Ω = 1 V/A.


When spelled out in full, unit names are treated like ordinary English nouns. Thus the names of all units start with a lower-case letter, except at the beginning of a sentence or in capitalized material such as a title. In keeping with this rule, the unit symbols for Ampere is a capitalized "A" and Volt is capitalized "V" because both unit names are based on the names of scientists. 

Andre Marie Ampere (1775 - 1836) Name endures in everyday life in the ampere, the unit for measuring electric current. These biographical website can help you learn more:

Alessandro Volta (1745 - 1827) Name endures in everyday life in the volt, the derived unit for measuring electric potential and also the inventor of the first battery. These biographical website can help you learn more:

Resources for Students and Teachers

graphic image of SI Superhero, Ms. Ampere
Credit: J. Wang and B. Hayes/NIST

League of SI Superheroes – Ms. Ampere

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. Ms. Ampere has a shocking amount of power over the flow of electrons—electrical current. In practical terms, an ampere is the measure of the flow of electrons past a point—about 6 quintillion electrons (that's a 6 followed by 18 zeros!) per second.

Navigate to more SI base unit information:



Created June 21, 2011, Updated January 16, 2024