LINES OF THE DIAGRAM.
Fig. 32.
[Fig. 32] represents an indicator diagram, such as might be taken from a locomotive with well-constructed valve-motion cutting off steam at about one-third of the stroke. In the figure, the line IJ represents the line that would be drawn by the pencil when no steam is admitted to the indicator piston; so it is called the atmospheric line. This is the neutral line of the diagram representing the position of the pencil when both sides of the indicator piston are exposed to the atmosphere: hence it is the base line from which pressures either above or below are measured. The line AB is the admission line, so called because its beginning A represents the point where the valve begins admitting steam to the cylinder: BC is the steam-line, usually considered as beginning at the point of positive change in direction of the admission line. Admission of steam does not cease till the point D is reached, but wire-drawing begins at C. The curve DE is the expansion line, and is traced while the valve is closed, and previous to the opening of the exhaust; the pressure falling by expansion. At E, release begins, and the exhaust line extends to F; from F to G is the line of counter-pressure, drawn during the return stroke of the piston and while the exhaust is open; and G to A is the compression line, drawn during the return stroke after the exhaust-valve has closed.