The Moving Coil Ammeter
The construction of a moving coil ammeter is shown below.
A rectangular coil to which a pointer is fixed is pivoted on a set of jewelled bearings between a soft iron cylinder and the cylindrical poles of a strong magnet. When a current flows the soft iron core becomes magnetised and forces are produced on the sides of the coil, according to the left hand rule. The forces are equal and opposite and constitute a couple. The coil rotates until the deflecting torque produced by the current and field is equal to the force exerted by the hairsprings.
The soft iron cylinder concentrates the flux in the space between the core and the magnet, so is constant whatever the position of the coil. This means that the torqu is proportional to the current and the scale can be made linear. The whole instrument is mounted on an aluminium former which results in eddy currents which oppose the movement of the coil so that it does not oscillate but quickly comes to a steady deflection whatever the current.