How can electric fields be used to detect cracks in metals? My physics teacher brought this up in a lecture and I am not exactly sure what he is saying.
 A: Cracks and other flaws can be found in metal parts by scanning the surface of the part with a small electromagnetic coil, which induces a current flow in the metal part as if the coil and the part were two coils coupled together in a transformer. the presence of a flaw like a crack in the part perturbs the flow of induced current and in turn upsets the current flow in the coil, which can be detected with a sensitive electronic circuit which thereby signals the test operator that a flaw is present at that point. This is called an eddy current test. 
If the metal part is ferromagnetic, surface cracks can be detected by temporarily magnetizing the part with a very strong external magnetic field and then rinsing the surface with a special fluid which contains a suspension of extremely finely-powdered iron in which the particles have been coated with a fluorescent dye. In the vicinity of a crack, the field lines in the magnetized part get bunched up and protrude slightly from the crack, attracting and entrapping the iron particles there. The test operator then rinses the excess fluid off the part and shines a UV light at it, which causes the dyed particles to visibly glow and trace out the crack. This test is called magnetic particle inspection (Magnaflux).
Cracks can also be detected in a flat metal part by attaching a piece of photographic film to it and then applying a brief high voltage pulse to the part. Electrons are ejected preferentially from the sharp edges of the crack and ionize the air nearby, causing it to glow faintly and expose the film. Once the film is developed and printed onto photographic paper, the cracks can be seen in it as bright lines. This is called corona discharge imaging. 
