Indicator Diagram

Instrument for studying the action of a fluid of varying pre ure and volume; devised by James Watt, and used practically for determining the work done by a steam engine. It has since been perfected so as to be applicable to many other cases in which a result is desired which is in any way dependent upon a product of two variables. It consists e entially of a sheet of paper and a pencil, which are made to have two relative motions at right angles to each other; the line traced upon the paper by the pencil and the area inclosed by it when referred to two rectangular datum lines, furnish means of calculating, in the case of a steam engine, e.g., the total work done during a stroke, and the pre ure or volume of the steam in the cylinder at any point of the stroke.