174 STEAM ENGINES 



to the motion of the piston. The outline then drawn by the 

 pencil, termed an indicator diagram, shows the pressure on 

 the piston at every point of the stroke. The slip of paper 

 on which the diagram is drawn is called an indicator card. 



Outside-Spring Indicator. The spring of the indicator 

 shown in Fig. 1 is subjected to the heat of the steam, and, 

 when highly superheated steam is used in the engine, the spring 

 may be rendered inaccurate by the heating due to the steam. 





FIG. 2 



An indicator with its spring located outside the barrel is shown 

 in Fig. 2. This form obviates the danger of heating the spring 

 and thus introducing eriors, and at the same time it has the 

 advantage of allowing the spring to be changed for a heavier 

 or a lighter one, with less trouble. In other details this indica- 

 tor is similar to that shown in Fig. 1. 



Indicator Sptings. The height to which the piston will rise 

 under a given steam pressure depends on the stiffness of the 

 spring. Indicators are usually furnished with a number of 



