SI Units – Electric Current

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 ∆ν_Cs.

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 are 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:

• IEEE Global History Network
• University of St. Andrews, Scotland

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:

• Engineering and Technology History Wiki
• Massachusetts Institute of Technology (MIT) Libraries
• Spark Museum of Electrical Invention

Return to Top of Page

Resources for Students and Teachers

• SI Base Units Relationship Poster (SP 1247). (NIST)
• SI Units Card Deck (SP 1297). (NIST)
• A is for Ampere. (Circuit Playground, Video)
• Electricity Basics: Voltage, Current, Resistance, and Ohm’s Law. (IDYL)
• Ampere's Law. (TeachEngineering, Grade 12)
• Ohm’s Law. (PhET Interactive Simulations)
• History of NIST Quantum Voltage Standards. (NIST)
• Top 10 Tips for Teaching the Metric System. (NIST)

Return to Top of Page

Credit: J. Wang and B. Hayes/NIST ©2020 U.S. Secretary of Commerce. All Rights Reserved.

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

• SI Units
• Fundamental Physical Constants
• Length – meter (m)
• Time – second (s)
• Amount of substance – mole (mole)
• Temperature – kelvin (K)
• Luminous intensity – candela (cd)
• Mass – kilogram (kg)

Return to Top of Page