All About Engines

Power and Its Measurement 3^5 throughout the whole of the stroke, and this means is provided by the steam engine indicator. Suppose we have a piston fitted in a small chamber connected by means of a pipe with the interior of the cylinder, as in Fig. 175, and held down by a spring. Then as the pressure in the cylinder varies, the piston will rise and fall in unison with it. A pencil, there- fore, attached to the little piston rod will make a straight line up and down on a piece of paper pressed against the point. If now this paper is caused to move in a direc- tion at right angles to that in which -the pencil is pointing, a more or less curved line, indicating the fall of pressure, will be produced. Moreover, if, at the end of the stroke, the paper moves in the oppo- site direction, the line drawn upon it will represent the pressure during exhaust. The vertical distance be- tween these two lines will represent the effective pressure upon the piston, assuming that it does not matter in Fig. 175.—To illustrate principle of indicator which end of the cylinder the pressures are measured. In the actual steam engine indicator, a good type of which is shown in Fig. 176 on Plate 30, there is a small piston in a vessel which is connected to the inside of the cylinder by means of a pipe, and the piston is held down by a spring which will be seen at the top of