The ampere, often referred to as the amp, is a unit of measurement for electric current. The term "electric current," often symbolized by the letter "I" in calculations involving current, refers to the rate of flow of electrical current, or the amount of electrical charge that passes a given point in a circuit in a certain amount of time. One ampere is equivalent to one coulomb per second, and one coulomb is equal to 6.24 x 1018 electrons, so one ampere is equivalent to 6.24 x 1018 electrons passing a given point in a circuit in one second. Coulombs and amperes are closely related in that the former represents the amount of charge running through a circuit while the latter represents the rate at which that charge flows.
The "ampere" is named after André-Marie Ampère, a French physicist and mathematician who conducted significant research in the area of electromagnetism. It belongs to the Système International d'Unités, or SI unit system, which is the standard international system of units used in science. This system of units is useful because most people working in science are familiar with them and because they tend to be fairly easy to understand and to convert when necessary.
Many other units of measurement with more specific purposes in science and industry make use of the ampere as a base unit. The ampere hour, for instance, is equal to 3600 coulombs, or the amount of charge built up from a steady one amp current running for one hour. This unit is often used to describe the charge capacity of batteries.
Electrical current is closely related to the ideas of electrical resistance, measured in ohms, and electrical conductance, measured in siemens. Resistance is a measure of how strongly a material impedes the flow of electrical current while conductance, conversely, describes how easily a current can flow through a material. In many cases, the current that can flow through a given material varies based on the temperature of the material.
An ammeter is a device that measures, in amperes, the flow of electrical current through a material. Different types of ammeters are made with different levels of sensitivity in order to accurately measure currents much higher or lower than one ampere. Ammeters have a variety of both scientific and commercial applications. They can be used in experiments concerning electricity and magnetism, as energy meters for power companies, or to estimate the amount of charge contained in batteries.