Electric current in the International System of Units (SI) is measured in amperes, denoted as $A$.

The ampere, often abbreviated to ‘amp’, is the standard unit for electric current in the SI system. This unit is named after the French physicist André-Marie Ampère and is essential for understanding the principles of electricity and magnetism.

Electric current refers to the flow of electric charge, which is typically carried by moving electrons in a conductor, such as a wire. The formal definition of the ampere specifies that it is the constant current that, when maintained in two straight, parallel conductors of infinite length and negligible circular cross-section, separated by one metre in a vacuum, produces a force of $2 \times 10^{-7}$ newtons per metre of length between these conductors.

In simpler terms, one ampere of current corresponds to one coulomb of electric charge—equivalent to approximately $6.242 \times 10^{18}$ charge carriers, usually electrons—moving past a specific point in one second. It is important to note that the direction of current flow is conventionally defined as the direction in which positive charges move. However, in metallic conductors, the actual charge carriers are electrons, which flow in the opposite direction. This distinction does not impact the calculations involving current.

The ampere is classified as a base unit in the SI system, meaning it is one of the fundamental units from which other units are derived. For example, the unit of electric charge, the coulomb, is defined as one ampere-second. Similarly, the unit of electrical power, the watt, is defined as one ampere multiplied by one volt.

In practical applications, electric current is frequently measured in smaller units such as milliamperes (mA) or microamperes (µA), particularly in the field of electronics. Conversely, in high-power applications such as power transmission, current may be measured in kiloamperes (kA).

Understanding the ampere and the measurement of electric current is crucial in physics, especially in the areas of electromagnetism and electronics. This fundamental concept underlies much of our modern technology, from the smallest electronic devices to the largest power stations.