THE AMPERE
 

The ampere, symbolized by the uppercase nonitalicized English letter A (or abbreviated as amp), is the unit of electric current. A flow of approximately 6.241506 × 10¹⁸ electrons per second past a given fixed point in an electrical conductor produces an electrical current of 1 A.
Various units smaller than the ampere are often employed to measure or define current. A milliampere (mA) is one-thousandth of an ampere, or a flow of 6.241506 × 10¹⁵ electrons per second past a given fixed point. A microampere (μA) is one-millionth or 10^-6 of an ampere, or a flow of 6.241506 × 10¹² electrons per second. A nanoampere (nA) is 10^-9 of an ampere; it is the smallest unit of electric current you are likely to hear about or use. It represents a flow of 6.241506 × 10⁹ electrons per second past a given fixed point.
The formal definition of the ampere is highly theoretical: 1 A is the amount of constant charge-carrier flow through two straight, parallel, infinitely thin, perfectly conducting media placed 1 m apart in a vacuum that results in a force between the conductors of 2 × 10^-7 newton per linear meter. There are two problems with this definition. First, we haven’t defined the term newton yet; second, this definition asks you to imagine some theoretically ideal objects that cannot exist in the real world. Nevertheless, there you have it: the physicist venturing into the mathematician’s back yard again. It has been said that mathematicians and physicists can’t live with each other and they can’t live without each other.