Because the magnetic field is polarised, the meter needle acts in opposite directions for each direction of current. A DC ammeter is thus sensitive to which way round it is connected; most are marked with a positive terminal, but some have centre-zero mechanisms and can display currents in either direction. A moving coil meter indicates the average (mean) of a varying current through it, which is zero for AC. For this reason, moving-coil meters are only usable directly for DC, not AC.