Watt's steam engine indicator

Watt's steam engine indicator, with box and 2 line diagrams

This instrument was introduced by engineers Boulton & Watt in 1796 for the purpose of assisting them in adjusting the valves of a steam engine. In Watt's patent of 1782 there is a diagram of the pressures throughout the stroke of an engine working expansively, but this is clearly the result of calculation. The instrument may have been developed from the vacuum gauge (see inv. 1876-1260). By the addition to such an apparatus of a pencil to the piston rod and a sheet of paper moved transversely to it -features attributed to John Southern, one of Watt's staff-the instrument became a recorder of the pressure existing in the cylinder at every point of the stroke, and gave a diagram from which the work done during the stroke was easily determined. The instrument was kept secret, and was entrusted only to the staff of the firm; that exhibited belonged to the agent in Manchester, while the diagram was taken in 1840 by Prof. E. Cowper.