Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 100

The eddy currents induced by movement generate a magnetic field which opposes the change in magnetic flux. This is Lenz’s law, a clarification upon Faraday’s law of induction. An example of this can be observed by introducing a sheet of non‐ferromagnetic metal such as aluminium or copper into the bore of the magnet. If you position the sheet vertically and transversely (normal to B₀) and then allow it to drop towards the horizontal, the flux from B₀ changes as the angle to B₀ increases and induced magnetic field will oppose B₀. The ensuing attractive force opposes the gravitational force and the sheet will tip in slow motion down towards the horizontal. Similarly, moving a non‐ferromagnetic conducting object in the fringe field gradient will result in resistance to that motion. Make sure the metal you use is non‐ferromagnetic.