Abstract: A screen is disclosed on the surface of which a certain number of resistant conducting emitter strips and insulated conducting receiver strips are laid, the emitter strips being parallel-mounted with the terminals of a voltage generator and the receiver strips being interlaid between the receiver strips such that a conducting receiver strip lies between two resistant emitter strips. The electronics of control comprises an exploring stage to control the receiver strip time-related exploration. Thus, when a sufficiently conducting object used to indicate a point, has touched the surface and bridges an emitter strip and a receiver strip, a higher potential is detected on the corresponding receiver strip and the ordinate Y is given by the strip order number--while the abscissa X is calculated from measuring the potential between each ends of the emitter strip and the object.

Abstract: A process of making cellulated glass beads by forming a mixture comprising glass particles and cellulating agent into nodules, heat treating the nodules to cause fusion of glass and evolution of gas from the cellulating agent to form cellulated glass beads, and cooling the cellulated glass beads to a rigid condition. The cellulating agent utilized has the characteristic of creating expansion forces due to the evolution of gas during heat treatment which are generated wholly or mainly while the glass viscosity is within the range 10.sup.6.5 to 10.sup.8.5 poises. The heat treatment is carried out with a quantity of cellulating agent at a maximum cellulation temperature which is not above the temperature corresponding with a glass viscosity of 10.sup.5.5 poises and for a time such that the beads immediately following the heat treatment are populated with cells, and after cooling have a bulk density of not more than 0.5 g/cm.sup.