Abampere: CGS Electromagnetic Unit of Current
The abampere (abA) is the unit of electric current in the centimetre–gram–second electromagnetic (CGS-EMU) system. It is defined so that one abampere flowing in each of two infinitely long parallel conductors separated by one centimetre produces a magnetic force of two dynes per centimetre of conductor length. By definition, 1 abA equals 10 amperes in the SI system.
Engineers studying historical telegraphy, electrodynamic experiments, or classic metrology literature often encounter abampere values. Use the Ohm's Law calculator to convert those readings into modern amperes for contemporary circuit design.
Definition and Conversion
In the CGS-EMU framework, electromagnetic quantities are derived from mechanical base units (centimetre, gram, second) without introducing separate electrical base units. The abampere is one such derived unit and is exactly related to the ampere by:
1 abA = 10 A.
Consequently, current values expressed in abampere can be multiplied by 10 to obtain amperes, while amperes are divided by 10 to convert to abampere. Because charge in the CGS-EMU system is measured in abcoulombs (1 abC = 10 C), integrating current over time remains straightforward when translating legacy data.
Historical Background
The abampere emerged in the late nineteenth century as scientists sought coherent electromagnetic units tied to mechanical measurements. The International Electrical Congresses of the 1880s and 1890s adopted CGS-EMU units for telegraphy and laboratory work, preceding widespread adoption of the practical units—ampere, ohm, and volt—that were later formalised in the SI system.
Although the CGS system eventually ceded to SI, understanding abampere-based literature remains important for interpreting classic experiments by Maxwell, Heaviside, and Lorentz. Many historical tables of electrical constants use abampere notation, and the unit occasionally appears in astrophysical plasma research where CGS equations persist.
Conceptual Foundations
Force-Based Definition
The abampere’s force law definition mirrors the modern SI formulation of the ampere but employs CGS force and distance units. Two conductors one centimetre apart carrying 1 abA produce a force of 2 dyn/cm, compared with the SI relation of 2 × 10−7 N/m for 1 A currents separated by one metre.
Relationship to Derived Units
Abampere pairs with other CGS-EMU units: abvolt for potential difference, abohm for resistance, and abfarad for capacitance. Converting an entire CGS circuit to SI requires scaling factors (for example, 1 abV = 10−8 V). Maintaining dimensional consistency ensures accurate translation of impedance and power calculations.
Magnetic Field Interpretations
In CGS-EMU, magnetic field strength (H) is measured in oersted and relates directly to current via Ampère’s circuital law expressed with abampere. This linkage simplifies Maxwell’s equations when written in CGS form, explaining the unit’s persistence in some theoretical treatments.
Applications
Historical Circuit Analysis
Historians and educators convert telegraph and early power system records from abampere to ampere to contextualise performance claims. The Ohm's Law voltage tool helps translate EMU voltage drops into SI equivalents.
Astrophysical Plasma Modelling
Some magnetohydrodynamic derivations still employ CGS units. Researchers convert abampere-based current densities into amperes per square metre before applying SI-based diagnostic instrumentation.
Metrology Education
Comparing abampere and ampere definitions illustrates the evolution of measurement systems. Students can replicate classic force experiments to appreciate how the SI redefinition anchored current to the elementary charge.
Importance and Modern Relevance
While the SI ampere now dominates practical measurements, legacy datasets and theoretical treatments still reference the abampere. Conversion fluency ensures compatibility when digitising archives or comparing classical derivations with modern instrumentation manuals.
Maintaining awareness of CGS-EMU units also aids interdisciplinary collaboration, especially when astrophysics, plasma physics, and electrical engineering teams share models. Mastery of the abampere demonstrates respect for the historical foundation of today’s electrical standards.