James Watt was a skilled and inventive image-maker.

One of his greatest inventions was a graphing instrument, his indicatrix. He created the device to measure the performance of the steam engine. This instrument was so valuable to him that he kept it secret for many years. On the left you see a vertical tube that looks like a bicycle pump. The tube contains a piston. As the pressure increases, the piston rises, as the pressure decreases, the piston falls. The pump is also attached to the drive shaft of the engine. As the drive shaft moves back and forth, the tube moves left and right. The drive shaft?s X position is proportional to the volume of gas in the chamber.

Attached to this mechanism is a pen. The pen traces a curve as the steam engine goes through its thermodynamic cycle. The x-axis of the graph is proportional to volume and the y-axis to pressure. Recall that the product of pressure and volume is work, and so the area inside the curve measures the work performed by the engine. This instrument allowed Watt to evaluate different engine designs, and to tune the engine to maximize it?s performance. It is easy to understand why he thought it was so valuable.