INDICATOR (Lat., pointer). In steam en gineering, an instrument to measure and to record by means of a diagram the pressure of the steam in the cylinder of an engine. The indicator was invented by .James Watt, of steam engineering fame, and the modern indicator is merely a structural modification of Watt's origi nal design. Concisely described, the modern steam-engine indicator consists first of a small cylinder having communication with the interior of the engine cylinder by means of a short pipe provided with a stopcock. In this small cylinder works a piston having a piston-rod extending out of the top of the cylinder. This piston is nor mally pressed down to the bottom of the cylinder by a spiral spring. but when steam is admitted below the piston by opening the stopcock pre viously mentioned, the pressure forces the piston upward against the spring, compressing it a greater or less amount. according to the steam pressure. The top of the piston-rod is attached to a lever which carries a pencil at one end.
This pencil bears against a slip of paper attached to a drum. which hack and forth on its axis fur a distance and '111111 mot ion which corresponds exactly to the stroke and movement of the engine piston. In operation the indicator
piston rises and falls with the rise and fall of the steam pressure in the (yliiider, and the pencil point is thus given a vertical movement whose magnitude correspond- to the intensity of the :steam pressure. .1t the '-alit,' time the rota tion of the drum gives the paper a horizontal movement under the pencil point. corresponding in magnitude and speed to the engine-piston movement, so that the result of the two opera Lions is the inscription of a loop-shaped diagram on the paper. These diagrams enable the steam engineer familiar with their use to calculate the power of the engine, to examine and adjust the actions of the engine valves, and to make certain inferences concerning the transformation of heat into work, and the influence of the metal of the cylinder on that operation. .1 concise treatise on the subject of indicators and indicator dia grams is Peabody. The Strum Engine Indica/Or 1 New York. 19001.