STEAM ENGINES 175 



helical spring for measuring the steam pressure, a lever 6 for 

 transmitting the motion of the piston to a pencil point c, and 

 a drum d that carries paper on which this motion is recorded. 

 The card e is held close to the drum by clips /, so that the pencil 

 can easily trace the outline of the diagram. The piston, shown 

 at g, must work in the cylinder as nearly frictionless as possible, 

 the spring h being the only resistance to the upward motion of 

 the piston. This spring is calibrated; that is, it is tested so as 

 to determine the pressures required to move the pencil to vari- 

 ous heights against the resistance of the spring. Hence, it is 

 possible to find the pressure in the cylinder by the position of 

 the pencil point. By turning a cock in the small pipe con- 

 necting the indicator with the engine cylinder, steam may be 

 admitted to, or shut off from, the cylinder of the indicator at 

 pleasure. When steam is admitted through the nipple *', its 

 pressure causes the piston g to rise. The helical spring h is 

 compressed, and resists the upward movement of the piston. 

 The height to which the piston rises should then be in exact 

 proportion to the pressure of the steam, and as the steam pres- 

 sure rises and falls the piston must rise and fall accordingly. 



To register this pressure, a pencil might simply be attached 

 to the end of the piston rod, the point of the pencil being made 

 to press against a piece of paper. It is desirable, however, to 

 restrict the maximum travel of the piston to about J in. while 

 the height of the card may advantageously be 2 in. To give 

 a long range to the pencil while keeping the travel of the piston 

 short, the pencil is attached at c to the long end of the lever b. 

 The fulcrum of the lever is at j, and the piston rod is connected 

 to it at k through the link /. The pencil motion is thus from 

 four to six times the piston travel. 



The indicator, however, not only must register pressures, 

 but it must register them in relation to the position of the pis- 

 ton. This is accomplished by means of the cylindrical drum 

 shown at d. This drum can be revolved on its axis by pulling 

 the cord m that is coiled around it. When the pull is released, 

 a spring on the drum spindle, inside the drum, turns the latter 

 back to its original position. If the cord is connected with some 

 part of the engine that has a motion proportional to the motion 

 of the piston, the motion of the drum also will be proportional 

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