Indicator

Entry

William McNaught’s indicator for steam engines consists of a tubular brass body and a movable barrel mounted on a bracket alongside the body. A string coiled around the bottom of the barrel is fixed to a moving part of the engine. At every stroke, the string turns the barrel and a watch spring coiled in the bottom of the barrel turns it back to its original position. A piece of paper is wound around the barrel and secured with a pair of clamps. In the body, a piston with piston rod is fixed beneath a spiral spring, which the piston, when forced upwards by the steam or sucked downwards by the vacuum, either compresses or extends. The cock at the lower end of the body is screwed into the cylinder cover, so that the indicator piston will move simultaneously with the cylinder piston and proportionate to the pressure of steam and the vacuum strength. A pencil with a knife hinge is attached to the piston rod of the indicator and the point of the pencil presses on the paper secured to the barrel. If the barrel did not move, the pencil would merely trace a vertical line on the paper, but the combined movements of barrel and piston trace a curve on the paper which is called the indicator diagram, which determines the nature of the engine’s performance.

McNaught’s specific improvement of James Watt’s indicator was the barrel,¹ whereas Watt used a simple flat writing pad.