The usual manner of testing the expansion curve of a diagram is to compare it with a curve representing Mariotte’s law for the expansion of a perfect gas.
A theoretic expansion curve that will accord with Mariotte’s law may be constructed on the diagram by the following method:
The diagram, as drawn by the indicator, will have the atmospheric line upon it, and from this as a basis we may mark in the line of no pressure or line of perfect vacuum.
To do this we draw, beneath the atmospheric line, a line as far beneath it as will represent the atmospheric line, on the same scale as the spring used, in the indicator, to draw the diagram.
Suppose, for example, that a 30 lb. spring was used, and assuming the atmospheric pressure to be 15 lbs. per inch, then the line of no pressure would be drawn half an inch below the atmospheric line, because 15 lbs. pull on the spring would cause it to distend half an inch.
The clearance line must then be drawn in, according to directions that have already been given.
Fig. 3369.
The next thing to do is to divide the length of the diagram into any convenient number of equal parts, by vertical lines parallel to, and beginning at, the clearance line, as shown in [Fig. 3369]. These lines are numbered as shown, ten of them being used because that is a convenient number, but any other number would do.