An instrument which draws a diagram showing the varying
   pressure in the cylinder of an engine or pump at every point of the
   stroke. It consists of a small cylinder communicating with the engine
   cylinder and fitted with a piston which the varying pressure drives
   upward more or less against the resistance of a spring. A lever imparts
   motion to a pencil which traces the diagram on a card wrapped around a
   vertical drum which is turned back and forth by a string connected with
   the piston rod of the engine. See Indicator card (below).