In practice, one must eliminate the effect of the earth’s magnetic
field and substitute very long wires by multiturn coils of appropriate
geometries An instrument called the current balance is used to measure
this mechanical force The SI unit of charge, namely, the coulomb, can now be defined in
terms of the ampere When a steady current of 1A is set up in a conductor, the quantity of
charge that flows through its cross-section in 1s is one coulomb (1C)