Failure of an instrument in a plant not only leads to unwanted shutdown of the plant, but also adversely influences plant safety and economy. By the redundancy and decision by majority system which employs a magnetic deviation detector and averaging unit, the reliability of plant instrumentation can be considerably improved. A description is given of its components, i.e., the magnetic deviation detector, magnetic operational applifier, magnetic ON-OFF switch in circuit, 2f -type magnetic amplifier for low-level signals, and magnetic multiplier-and-divider. The principles are first explained and then the results of experiments with the system are presented. It has been found that the system gives very satisfactory results.