Why is the Ampère an SI Base Unit but Not Voltage?

The ampère (A), a fundamental unit of electric current in the International System of Units (SI), serves as one of the seven base units. In contrast, voltage or electric potential difference, also known as potential difference, is defined as a derived unit. This article explains the distinctions between these two important units and discusses the historical context of their definitions.

Ampère: An SI Base Unit

The ampère is one of the seven base units in the SI system. It is defined as the constant current that, if maintained in two straight parallel conductors of infinite length and negligible circular cross-section, would produce a force of 2times;10^{-7} newtons per meter of length between those conductors. This precise definition establishes the ampère as a fundamental building block for other units of measurement within the SI framework.

Voltage: A Derived Unit

Voltage, or electric potential difference, is defined as the amount of electrical potential energy per unit charge. It is measured in volts (V), which is a derived unit. Specifically, 1 volt is defined as 1 joule per coulomb. Here, a joule is a unit of energy, and a coulomb is a unit of electric charge. This relationship indicates that voltage is dependent on other measurements, rather than being a fundamental quantity by itself.

Historical Context and Practical Measurement

In 1893, the International Electrotechnical Commission (IEC) established volt, ohm, and ampere as standard units based on physical measurable processes. For instance, a 1-metre mercury column was used to standardize the ohm. The coulomb, a much higher unit than the basic electron charge, was challenging to measure due to the slow and unreliable process of electrolytic silver deposition. Instead, the ampère was defined through a direct effect measurement: the force between two carrying parallel current conductors. This measurement involved a sophisticated balance and was adopted by the International Committee on Weights and Measures in 1948 and integrated into the SI system in 1960.

Summary: Fundamental and Derived Units in the SI System

The ampère and voltage play distinct roles in the SI system. The ampère is a fundamental unit of electric current, based on a well-defined and reproducible physical phenomenon. In contrast, voltage is a derived unit that depends on the ampère and other units. Understanding this distinction is crucial for the accurate and consistent application of electrical measurements.