Ampere

The ampere (symbol: A) is the unit of measurement for electric current and is one of the seven base units of the International System of Units (SI). Electric current refers to the amount of electric charge passing through a given point per unit of time.

One ampere is equivalent to one coulomb of charge passing through a point in one second. The unit is named after the French scientist André-Marie Ampère, who contributed to the study of the relationship between electric current and magnetism.

Modern definition

Historically, the ampere was defined by the magnetic force produced between two parallel conductors carrying current. Following the 2019 redefinition of the SI, the ampere is now defined using a fixed numerical value of the elementary charge (e), measured in coulombs. As a result, the definition is based on an invariant numerical constant rather than on a measurable artifact.

Relationship to other units

The ampere is directly related to the volt and the ohm as described by Ohm's law: current in amperes equals voltage in volts divided by resistance in ohms. Electric power, measured in watts, is derived from the product of voltage and current.

Practical use

The ampere is used to measure the capacity of electrical devices, household fuses, and circuits. Smaller units are also commonly used, such as the milliampere (mA, one thousandth of an ampere) and the microampere (µA). The instrument used to measure current is called an ammeter, which is connected in series so that current flows through it.