Abstract: A process and device for focussing a light energy beam on a data carrier comprising sites formed of virgin areas free from any data recording provided along tracks and serving as reference plane for said focussing. In a preferred variation, these areas are preceded by flags associated with a specific code. Control is achieved by measuring the deviation between the plane of focussing of a beam into a spot scanning the tracks and the reference plane formed by the surface of the virgin areas. Between two passages of these areas the measurements are stored. The focussing error signal is obtained by filtering these measurements.

Abstract: The angular velocity of an information element contained in the precut track of an information carrier disk is controlled by means of a concentric circular ring located internally with respect to the precut track and constituted by mark zones uniformly spaced in angular sectors. A light beam is projected onto the circular ring and the emergent beam is modulated as a function of the mark zones and blank zones which form the circular ring. Errors arising from eccentric displacement of the track can thus be corrected and the rate of travel of information can be controlled in relation to the reading or recording light beam.

Abstract: A mobile pre-etched data carrier is provided in which the pre-etching is formed by a succession of discrete non continuous elements materializing the mean axis of the tracks. The spatial distribution of these discrete elements may be uniform or not. In a first embodiment, each discrete element is formed by a smooth track section. In a second embodiment, each discrete element comprises several sections defining a particular code. In other preferred variations of the invention, each element comprises, besides a portion centered on the mean axis of the tracks, one or more portions offset with respect to the axis. Finally, each of the pre-etchings may itself be preceded by an additional pre-etching element used for the purpose of synchronization. The tracking device comprises photodetector means, measuring circuits comprising sampler-inhibitors or storage integrator circuits and sampling circuits. A radial tracking error is formed from the evolution of the signals detected during successive passages.

Abstract: The invention relates to a process and apparatus for ridding the worn anode butts of electrolysis bath crusts which adhere to them.The apparatus essentially comprises a jack supporting a pneumatic drill equipped with a percussion tool. The jack is supported by a movable and orientatable arm.Application to the recovery of worn anodes from igneous electrolysis cells for the manufacture of aluminium.

Abstract: This invention deals with under water towing from a ship having a central ll, the side walls of which are provided with arcuate slideways on which a carriage may run. This carriage has a yoke from which it supports a first pulley over which the tow cable is passed in its run between the thing towed and a second pulley located inside the ship. The yoke is pivoted to the carriage on the same axis as that of the cable run between the first and second pulleys so that the first pulley position is maintained against ship's roll. The carriage moves on the slideways so that the cable run between the two pulleys stays of equal length as the swell of the sea lifts or lowers the ship relative to the thing towed. The whole carriage -- first pulley assembly can be drawn into a well-head frame which can be tilted to free the well-head from obstruction.