Abstract: In a computer controlled automatic plotting device, an ink level detector is provided in a plotter pen cartridge. An ink-following object is provided in the plotter pin cartridge and is supported by the writing fluid. The ink-following object carries a magnet. As the ink-following object traverses down the inside of the plotter pen cartridge, the position of the magnet triggers a sensor at a predetermined level. The sensor then effects a visual or audible signal to alert an operator that the cartridge is running out of writing fluid. Alternatively, the sensor activates a control system to shut down the plotting device to permit replacement of the plotter pen cartridge with one having a new ink supply.

Abstract: A pressurized roller pen includes a thixotropic viscoelastic ink that is a gel at rest, but becomes a thin liquid under the shearing action of the pen's revolving ball-point. This ink includes a polyacrylic resin polymer, at least one highly polar solvent, at least one solvent of low or medium polarity, and colloidal silica, together with pigments, dyes, surfactants and/or pH modifiers.

Abstract: An erasable ink for use in a ball point pen, particularly in a pressurized cartridge, is a fluid comprising at least 10% w of polyvinyl acetate or a copolymer of ethylene and vinyl acetate, and at least 10% w trichloroethylene. The ink preferably also contains at least 5% w of a solvent boiling in the range of 180.degree.-250.degree. C., a plasticizer boiling above 270.degree. C., a colorant, and a tackifier.

Abstract: An erasable ink for use in a ball point pen, particularly in a pressurized cartridge, is a fluid comprising at least 10% w of polyvinyl acetate or a copolymer of ethylene and vinyl acetate, and at least 10% w trichloroethylene. The ink preferably also contains at least 5% w of a solvent boiling in the range of 180.degree.-250.degree. C., a plasticizer boiling above 270.degree. C., a colorant, and a tackifier.