Another method of drawing this curve is shown in [Fig. 3370]. Having drawn the clearance line b c, and vacuum line d c, as before and chosen where the curves shall touch (as at a), then draw from a a perpendicular a a.
Draw line a b, parallel to the vacuum line, and at any convenient height above or near the top of the diagram.
From a draw a c, and from a draw a b parallel to d c, then from its intersection with a c, erect the perpendicular b c, locating on a b, the theoretical point (c) of cut-off.
From a number of points on a b (which may be located without regard to equally spacing them), such as e, f, g and h, draw lines to c, and also drop perpendicular lines, as e e, f f, g g, h h.
From the intersection of e c with b c, draw a horizontal line to e. From the intersection of f c with b c, draw a horizontal line, and so on; and where these horizontals cut the verticals (as at e, f, g, h) are points in the curve, which begins at c, and passes through e, f, g, h, to a.
But this curve does not correctly represent the expansion of steam. It would do so if the steam remained or was maintained at a uniform temperature; hence it is called the isothermal curve, or curve of same temperature. But in fact steam and all other elastic fluids fall in temperature during their expansion, and rise during compression, and this change of temperature slightly affects the pressure.
A curve in which the combined effects of volume and resulting temperatures is represented is called the adiabatic curve, or curve of no transmission; since, if no heat is transmitted to or from the fluid during change of volume, its sensible temperature will change according to a fixed ratio, which will be the same for the same fluid in all cases.
A sufficiently close approximation to the adiabatic curve to enable the non-professional engineer to form an idea of the difference between the two may be produced by the following process:
