Ferroprobe

an instrument for measuring and indicating magnetic fields (principally steady or slowly varying fields) and their gradients. The operation of a ferroprobe is based on the change in the magnetic state of a ferromagnetic material under the influence of two magnetic fields having different frequencies.

In its simplest version, a ferroprobe consists of a ferromagnetic core on which there are two windings—an excitation winding supplied with an alternating current and a measurement winding. When no measurable magnetic field is present, the magnetism of the core is reversed over a symmetrical cycle by the action of the alternating magnetic field created by the current in the excitation winding. The change in the magnetic flux produced by the magnetic reversal of the core along a symmetrical curve induces an emf in the measurement winding that varies according to a harmonic rule. If a measurable, steady or slightly varying magnetic field acts on the core at the same time, the curve of the magnetic reversal changes its size and shape and becomes asymmetrical. The value and the harmonic content of the emf induced in the measurement winding are thereby altered. Specifically, even-harmonic components appear in the emf that have a value proportional to the intensity of the measured magnetic field and that are absent during a symmetrical magnetic-reversal cycle.

A ferroprobe usually consists of two cores with windings that are interconnected in such a way that the odd-harmonic components are practically compensated. The measuring apparatus is thus simplified, and the ferroprobe’s sensitivity increased. The most common ferroprobes have the following major subassemblies: an AC oscillator to supply the excitation winding; a filter for the odd-harmonic components of the emf, which is connected to the output of the measurement winding; an amplifier for the even harmonics; and an output measuring instrument. Ferroprobes have a very high sensitivity to a magnetic field—up to 10^–4 or 10^–5 amperes per meter.

Ferroprobes are used to measure the earth’s magnetic field and its variations (especially in prospecting for commercial minerals that create local anomalies in the geomagnetic field) and the magnetic fields of the moon, the planets, and interplanetary space. They are also used to detect ferromagnetic objects and particles in a nonferromagnetic medium (especially in surgery) and in quality-control systems for manufactured products.