Fig. 3362.
Both are made with the piston and parallel motion as light as possible, in order to enable the taking of diagrams at as high a speed of engine revolution as possible.
Each consists of a cylinder and piston, the bottom surface of the latter being in communication with the bore of the engine cylinder, so as to receive whatever steam pressure there may be in the cylinder.
This indicator piston receives, on its upper surface, the pressure of a spiral spring, which acts to resist the steam pressure.
The indicator piston rod actuates an arm or line on the end of which is a pencil, which, by means of a parallel motion, is caused to move in a straight line.
Fig. 3363.