"Coffin" Averaging Instrument and diagram holder

"Coffin" Averaging Instrument and diagram holder

This is a special form of planimeter arranged for measuring steam engine indicator diagrams. Starting from a point in the toe of the diagram, and with the measuring wheel at zero, the tracing point is led round the diagram when, on completing the circuit, the fresh reading on the wheel is the area of the diagram in square inches. By now sliding the tracing point vertically up the diagram, until the measuring wheel again reads zero, a height is obtained equivalent to the mean ordinate, and when measured with the indicator scale gives the average mean pressure throughout the stroke.