Crosby Steam Gage & Valve Co.steam engine indicator, 1899, used by S. Z. de Ferranti Ltd Engine Department.

Steam engine indicators were used by the mill engineer to monitor the pressure inside one end of the cylinder. Paper would be secured around the drum and the rotating drum is attached to a moving part of the engine with a piece of string. As the engine runs, the paper is rotated under the pressure indicator's pencil, causing it to draw a line as the drum moved back and forth, creating a visual representation of the pressure inside the cylinder during one complete stroke of the piston.