The engine Indicator is not used much at all these days, but it used to be a major tool for use by steam and diesel engine manufacturers to enable
them to work out horsepower ratings for the products. They are also a useful diagnostic tool to show problems with injection, valves, leaking piston rings etc.
They work on the principle of showing cylinder pressure plotted against piston movement, and the resulting trace is marked onto a piece of treated paper to
give a permanent record. The pressure is taken from the cylinder on test into the indicator. The indicator has a small cylinder with a piston which operates
against a spring which sets the operating range.
The Crosbie Steam Gage & Valve Co. Indicator
The following picture shows a typical instrument of the early 1900's, based on the Crosby Steam
Gage & Valve Co type. The height is about 8" overall.
A more detailed sectional view follows this picture.