Unit of electric current. The ampere
The ampere is the current which, if flowing in two straight parallel wires of infinite length, placed 1 metre apart in a vacuum, will produce on each of the wires a force 2 x 10 – 7 newton per metre length. The manner in which the current sets up this force is explained on page 426. As far as the practical measurement of current is concerned we do not pass the current through two very long wires and measure the force between them as it is not possible to do this with any reasonable degree of accuracy. Instead, we apply the definition to calculate the force between two circular coils of wire in an instrument called a current balance. By measuring the force between the coils, which can be done very accurately, we are then able to determine the value of the current flowing through them in amperes.
Current balances are large and expensive pieces of apparatus and are used only in standardizing laboratories such as the National Physical Laboratory (Fig. 35.l). The ammeters (page 444) used in ordinary laboratories for measuring current have been
calibrated so as to agree with the readings of a standard current balance. The sub-units of current commonly used are the milliampere (1 mA = 0.001 or 10-3 A) and the microampere [l uA = 0.000001 or 10-6 A).