INDICATOR, in steam engineering, an instrument invented by James Watt, to record, graphically and automatically, the pressure in an engine cylinder at•every point of the stroke. By means of the diagram that the indicator draws, it is possible to determine whether the valves of the engine are working correctly or not, and it is also possible to estimate the horse power that the engine is developing with con siderable accuracy. Commercially, the engine indicator may be had in many forms; but all are based on the same fundamental principle, which will be understood by refer ence to the diagram presented in Fig. 1. The paper upon which the indicator diagram is to be drawn is here supposed to be secured, flat, to a carriage which travels back and forth upon a track; the motion of the carriage correspond ing precisely to the motion of the piston of the engine. In practice it would be inconvenient to have the carriage travel a distance equal to the whole stroke of the engine, and hence some form of reducing motion is used, so that the motion of the carriage may follow the motion of the engine piston accurately, but frith Ma daltn terially re uced velocity. The indicator'• 10 drawn by means of a pencil-point on k the piston rod of a small steam cylinder di 13 situated below the carriage, and which opens freely into the eylinde,r of the engine, ftont which the diagram is to be taken. The piston of the indicator is pressed downward by means of a spring whose strength is accurately known, so that the increase of pressure corresponding to a rise of one inch in the position of the pen cil-point is known. When the indicator is in operation, the pencil rises and falls propor tionately to the pressure of the steam in the engine cylinder, and the carriage, with its at tached paper, travels back and forth horizon tally, at the same time, keeping pace precisely with the motion of the piston of the engine. Under these circumstances the pencil-point traces a diagram somewhat like that shown in the illustration. In practice, the paper upon
which the diagram is drawn is usually wrapped about a cylindrical drum, which by rotating pre sents a new surface of paper for a new dia gram. It follows the motion of the engine piston just as the carriage here shown. In Fig. 2 an enlarged view of an indicator card (or diagram) is given, together with the tech nical names of some of its more important parts. The arrows show the direction in which the pencil travels as the diagram is drawn. The 'admission line° is the part that is drawn while the engine is in full communication with the boiler, and drawing steam from it. The angle marked "cut-off' corresponds to the instant at which the steam supply is cut off, and the expansion of the steam begins. The •expansion line' is drawn during the expansion of the steam; and on the return stroke, after the exhaust valve has opened, the "exhaust line° is drawn. When the exhaust valve has closed again, the steam remaining in the engine cylinder is compressed until the end of the stroke, the indicator meanwhile drawing the •compression line.* The °atmospheric line" is the straight, hori zontarline drawn by the engine when the con nection is broken between the engine cylinder and the indicator cylinder, and the latter is open freely to the air. The "vacuum line' is the line that would be drawn by the instrument under like circumstances, if a perfect vacuum could be maintained in the indicator cylinder. The 'vacuum line,' of course, must be drawn in by hand. It is parallel to the"atmospheric line,' and at a distance below it corresponding to a pressure of about 14.7 pounds per square inch, on the scale to which the diagram is drawn. The diagram shown in Fig. 2 refers to a condensing engine. In a non-condensing en gine, the exhaust line would not be lower than the atmospheric line, and would, in fact, be above it, if there were any sensible back-pres sure in the engine during exhaust.