Patent ID: 7541904

Claim:
A magnetic field generation source comprising: a yoke, connected with an upper press plate and a lower press plate, the lower press plate and the upper press plate being oppositely arranged; and a pair of magnetic field generating sources and pole plates that are oppositely mounted on the respective press plates and are arranged to generate a magnetic field between them; wherein each of said magnetic field generating sources includes a magnetic field adjusting device; the magnetic field adjusting device comprises adjusting bars of a soft magnetic material mounted at the periphery of the magnetic field generating source; the installed position of adjusting bars when mounted at the periphery of the magnetic field generating source is moveable in a direction substantially parallel to a magnetic field produced by the magnetic field generating source; the resulting magnetic field generated by the magnetic field generating source is adjusted as a function of the installed position of the bars; and said adjusting bars have a rack structure, and are arranged to be driven by means of mating pinion gears.