plate 2 and the edge of the container 3 could of course also be obtained by providing small projections on the underface of the plate since the annular space between the container and the outer bowl communicates with a rotating quantity of fluid at its middle where the pressure is low viz through the opening 7 the liquid level in the annular space will fall until a state of equilibrium is obtained electrolyte penetrating between the sample and the plate 2 will join the electrolyte escaping between the plate and the upper edge of the container and will flow into the annular space and from there into the container through the opening 7 since the liquid level in the annular space is lower than that in the container the latter being determined by the plate 2 the edge of the bowl 5 may be situated at a lower level than the upper face of the plate 2 and it is therefore possible to apply large metal samples extending beyond the contour of the apparatus the gases liberated by the electrolysis escape through a hole 15 of the plate 2 this hole also permits the liquid level in the inner container to adjust itself freely in the electrolytic process the metal sample forms one pole electrical connection being obtained by means of a spring clip 16 and the container 3 forms the other pole if a plurality of spring clips are provided in analogy to 16 the plate 2 may have a plurality of polishing openings.