Abstract: A self-cleaning dentist's mirror is disclosed which includes a hollow handle having a housing at one end and a mirror rotatably supported in the housing on a spindle. Turbine blades are attached to the bottom of the mirror, and the handle is connected to a suction device that draws air into the housing past the edges of the mirror and the turbine blades to rotate the mirror and remove water and other foreign substances from the mirror surface.

Abstract: A bipolar converter station for conversion between alternating current and high-voltage direct current comprises a first and a second active filter (AF11, AF12), each with its own piece of control equipment (CE11, CE12). The filters are intended for reduction of harmonics in a dc link, connected to the converter station, with a first and a second pole line (PC1, PC2), and each filter generates, in dependence on a sum of harmonic currents in the respective pole line, an influencing quantity (IF11, IF12) and supplies this to the dc link. The control equipment comprises frequency-selecting means (BP, CTR) which influence the frequency content of the respective influencing quantity to lie substantially within one of a first and a second of at least two predetermined, mutually non-overlapping frequency bands (FR1, FR2).