Abstract: A method of making measurements on a body includes the steps of placing a body in a space which is divided along a plane into adjacent regions of contrasting illumination with the dividing plane intersecting the body, rotating the body relative to the dividing plane about an axis and sensing the position on the dividing plane of at least a part of the exterior surface of the body with a sensor sensitive to the illumination at a plurality of rotational positions of the body. The axis about which the body is rotated lies in the dividing plane and light sources which divide the space into adjacent regions of contrasting illumination are located on the dividing plane.

Abstract: An electric discharge apparatus, for example a gas laser or discharge lamp, includes first and second electrodes defining a first discharge path and third and fourth electrodes defining a second discharge path alongside the first. A respective impedance is associated with any electrical connection between electrodes. The electrodes are electrically arranged such that either the current flow of a discharge along the first path is so different from the current flow of a discharge along the second path that, overall, the discharges repel one another, or stable electric discharges which coalesce are generated between respective electrodes of the first and second sets.A multi-electrode plasma torch is also provided comprising an electrode holder from which a plurality of electrodes project and a pilot electrode common to more than one of the electrodes.