CATALOG OF THE MECHANICAL COLLECTIONS gg 



ENGINE INDICATORS 



An engine indicator is an instrument for graphically recording the 

 pressure within an engine cylinder at every instant during the stroke 

 of the piston. The diagram produced by the indicator is used by 

 engineers to check many features of both the design and performance 

 of the engine. From it may be ascertained the amount of work done 

 by the steam or gas upon the piston ; the exact part of the stroke at 

 which each valve event occurs; the average pressure in the cylinder 

 during the stroke ; and many other facts of interest and value. 



The indicator is one of the many inventions attributed to James 

 Watt, who devised it primarily to measure the work done by his steam 

 engines. Watt was paid for his engines on the basis of the work that 

 they did as compared to the number of horses that they displaced, and 

 before the invention of the indicator his customers' low estimates of 

 the efforts of his engines caused him to complain that "the power of 

 a horse is growing to that of an elephant." 



The first form of the indicator was a modification of the vacuum 

 gauge, which was connected to the cylinder of the engine and was 

 read by an observer at any required inteiTals during the ponderous 

 stroke of the then slow-speed engines. The important addition of a 

 pencil attached to the pointer of the instrument, and so arranged that 

 it would register its position on a sheet of paper fastened to a flat 

 board that moved in synchronism with the engine piston, is believed 

 to have been suggested by John Southern, one of Watt's assistants. 



At just what date the first indicator was used is not known. Watt 

 found it of so much assistance to him in properly adjusting the valves 

 of his engines that he kept it secret as long as he could. The story 

 is told that an indicator was accidentally packed with an engine sent 

 from the Watt factory to Holland, where it subsequently fell into the 

 hands of an agent of a competitor and through him was revealed to 

 the world. 



The reciprocating drum to carry the paper, which is a part of every 

 present-day indicator, was introduced about 1825 to 1830 by Jolin 

 McNaught of Glasgow. McNaught's indicator retained the fixed con- 

 nection between the piston rod of the indicator and the pencil, which 

 required that the piston move as far as the pencil must travel to pro- 

 duce a diagram large enough to be legible. Prof. C. B. Richards, of 

 Connecticut, in 1862 designed the first indicator in which a compara- 

 tively short movement of the indicator piston was multiplied many 

 times by suitable linkage to produce the required movement of the 

 pencil. By thus reducing the movement of the piston its linear speed 

 was also reduced, and tne errors and distortion of the diagram occa- 

 sioned by the inertia of the indicator parts at high speed were partially 

 eliminated. The Richards is considered the first of the modern type 



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