MT is a non-destructive testing is the collective name given to repeatable processes applied to components or structures to interrogate their integrity, in such a way as to not impair their ability to operate or perform.
At the turn of the 20th century, such tests were crude, relatively insensitive and not widely used. There was eyeball examination aided by low-power magnifying glasses. Also, hearing tests were performed by tapping metal parts with a hammer and listening for any difference in the ringing sounds produced.
In the early 1920s, X-ray, which previously had been used only by physicians, was shown to be applicable to inanimate materials. While radiography can detect subsurface or interior defects in metals, a reliable, inexpensive testing method was greatly needed to locate surface cracks and discontinuities in high quantities of machine and structural parts.
That method was magnetic particle testing, based on the principle of using a piece of magnetized metal and applying magnetic particles to find invisible defects, without harming the part in any way.
Magnetic particle inspection is a nondestructive test method that provides for the detection of linear, surface and near surface discontinuities in ferromagnetic (material that can be magnetized) test materials.