Abstract: An apparatus and method are provided for sealing an outer surface 170 of a shaft 175 to an inner surface 165 of a hub 160. The seal includes a magnet 155, with top and bottom pole pieces 260, 265, and a ferrofluid 270 magnetically held in a gap 275 between the pole pieces 260, 265, and the hub 160 or the shaft 175. The top pole piece 260 has a cross-section that is L-shaped, with a horizontal portion 260a parallel to the magnet 155 and a shorter vertical portion 260b facing the shaft 175. In one version, the vertical portion 260b or the portion of the shaft 175 facing it is contoured to provide a flux gradient that axially concentrates the ferrofluid 270 in the gap 275. In another aspect, a catcher 335 is provided to reduce loss of ferrofluid 270 when the ferrofluidic seal 185 used to form an outside seal. In yet another aspect, a stationary ferrofluidic seal 345 is provided for sealing a stationary shaft 350 to a rotating hub 355.

Abstract: A motor is provided comprising a rotor, a stationary sleeve disposed about the rotor, a fluid dynamic bearing between the rotor and sleeve, and a limiter for restricting axial movement of the rotor relative to the stationary sleeve.

Abstract: A measurement module projects into a microscope a fiduciary image having a visually extensive area. Measurements are made by translating the fiduciary relative to the object between two positions in which different fiduciary image boundaries are tangent to respective object image sides, as determined by null positions in which neither dark overlap nor bright separation zones appear between the two semi-transparent images. The width of the object can be determined from the distance of relative movement of the fiduciary between these positions. In preferred form the fiduciary image has first and second boundaries in fixed relation at a predetermined spacing and the measurement is corrected by the spacing between the two fiduciary boundaries.