The magnetic inverter is a low-level magnetic amplifying device suitable for the input of multi-stage magnetic amplifiers requiring a high sensitivity. The principle of its operation differs from the conventional magnetic amplifier in that the load current corresponds to the even-harmonic m.m.f.'s which are present in any asymmetrically-excited iron-core system.It is found that a capacitor in parallel with the load can increase the gain by a factor of about 15, but that this occurs at the expense of linearity for large input signals.The inverter can have power gains ranging up to about 1 000 with corresponding time-constants of about 1 sec when operating at 50 c/s, whilst the zero stability is well within 10−12watt.The inverter is a valuable addition to measuring and control instruments, and its mode of operation forms an interesting comparison with the theory of the 100% self-excited transductor.