Indicator

Instrument first designed by James Watt for measuring and recording the pre ure in a steam engine cylinder at each point of a stroke. The e ential parts consist first of a cylinder, in which fits a piston, usually of an area equal to sq. in. This cylinder is secured by suitable nipples, couplings, and cock to one end of the engine cylinder, so that the working steam acts on its lower side, to push it out. This pre ure the spring is more or le compre ed. This record is taken upon a card wrapped around a drum, the latter being rotated nearly through a revolution by a cord whose free end is connected by a reducing device to the cro -head of the engine. A curve upon this card is traced for each revolution of the engine, and shows the succe ion of pre ures at that end of the cylinder. From this diagram, knowing the scale of the spring, the mean steam pre ure can be ascertained, and the horse-power computed. The diagram also shows defective valve-working, theoretical steam consumption, amount of back-pre ure in exhausting the steam, lo es by inadequate steam pa ages, etc. In condensing engines the line of back pre ure falls below the line of no pre ure on the spring (atmospherie line) so that the spring is extended to an amount which measures the amount of vacuum in the condenser.