Abstract: In an arrangement for the generation and focussing of shock waves for purposes of comminution of concrements, wherein the generation is provided by a pair of electrodes, and the focussing is provided by a rotational ellipsoid; the electrodes extend from a sleeve being inserted in the body of the ellipsoid such that the electrodes center on one of the two focal points of the rotational ellipsoid; a positioning device for the electrodes includes a position indicator on the electrode sleeve such as a metal or optical marking on a mechanical actuator having a highly accurately predetermined positional relation to the electrodes; a position sensor on the body of the rotational ellipsoid has a highly accurately determined position in relation to the first focal point; and a circuit is connected to the position sensor to provide a audible and/or visible representation if, and only if, the position indicator has an accurate position related to the sensor as an indication that the electrodes are centered on the first

Abstract: The contact pole of a trolley bus carries at its normally upper end a trolley shoe and an upwardly projecting guide which may jointly pivot on the pole about an approximately vertical axis. When the pole is partly raised by a heavy spring and swung horizontally, the trolley shoe is vertically aligned with an overhead wire when the guide abuts against the wire. When the pole is further raised by the spring, the trolley shoe engages the wire, and the guide is automatically dropped below the trolley shoe so as not to collide with hangers for the overhead wire. When the pole is lowered by an operating mechanism against the biasing force of the spring, a motor-driven element of the mechanism connected to the guide by a motion transmitting train automatically returns the guide to its position above the trolley shoe.