amperes

The ampere (symbol A) is the SI unit of electric current, defined as the flow of electric charge per unit time. In practical terms, the ampere is the current that, if sustained for one second, would transfer a charge of one coulomb. Since 2019, the ampere is defined by fixing the elementary charge e to exactly 1.602176634×10^-19 coulombs, making the ampere a fundamental-constant-based unit. The ampere is realized experimentally through quantum electrical standards that relate voltage, resistance, and current to fixed constants.

The unit is named after André-Marie Ampère, a French physicist who contributed to the development of electromagnetism. Submultiples and multiples are commonly used in practice, including the milliampere (mA, one thousandth of an ampere) and the microampere (µA, one millionth of an ampere), as well as the kilampere (kA) for larger currents.

Measurement and realization information: the ampere is realized in metrology laboratories using quantum standards that connect electrical quantities to fundamental constants. In everyday use, current is measured with devices such as ammeters, which are placed in series with a circuit. Typical instruments include galvanometers and digital multimeters, with high-current measurements often employing shunt resistors to convert current to a measurable voltage.

Applications and context: the ampere is central to all electrical circuits and systems, from microelectronic sensors that draw microamps to power distribution networks and electric motors that operate at amperes or kiloamperes. Understanding current, along with voltage and resistance, is essential for applying Ohm’s law and for designing and operating electrical devices safely and efficiently.