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.