242 



AUTOGRAPHIC DIAGRAMS. 



compare it with similar curves having rectangular co- 

 ordinates. The magnification of the extensions used in 

 this case was 130 to 1. 



132. Uses and Limitations of Autographic Diagrams. 

 Autographic stress-strain diagrams are sometimes described 

 as being analagous to indicator diagrams taken from fluid- 

 pressure engines, but this is an unfair comparison. The two 

 cases are not quite similar. An indicator diagram is taken 

 from an engine for two purposes, one qualitative and the 



POUNDS PER SQUARE INCH 



FIG. 119. Automatically drawn elastic load strain diagram of a 

 sample of Blaenavon Cast Iron. L is the curve for the first load- 

 ing, M for the second, and N for the third. 



other quantitative. It is required to determine the work 

 which is being performed upon the piston of the engine 

 during a cycle or number of cycles ; and, at the same time, 

 the diagram is made use of for the purpose of finding out 

 the manner in which the parts of the engine are doing 

 their work. No other means are available for determining 

 these two sets of facts the time taken to complete the 

 cycle of an engine is relatively short ; and, therefore, it is 

 for this reason not possible to take a number of isolated 

 readings, and to plot the complete diagram afterwards from 

 these. The indicator must be relied upon almost entirely, 

 and there is no appeal against its decision. 



