These analyses are made by means of calorimeters which give the calorific value either at a constant pressure or at a constant volume.
Constant-volume instruments give a somewhat weaker record than constant-pressure instruments; but according to Professor Aimé Witz, the inventor of an excellent calorimeter, the constant-volume type is almost indispensable in gaging the efficiency of explosion-engines.
[Illustration: FIG. 151.--Record made when effective load was changed at two different intervals.]
[Illustration: FIG. 152.--The Witz calorimeter.]
=The Witz Calorimeter.=--The accompanying diagram (Fig. 152) illustrates Professor Witz's instrument. Its elements are a steel cylinder having an interior diameter of 2.36 inches, about a thickness of 0.078 inch and a height of about 3.54 inches, so that its capacity is about 15.1 cubic inches, and two covers screwed on the cylinder to seal it hermetically, oiled paper being used as a washer. The upper cover carries a spark-exciter; the lower cover is provided with a valve which discharges into a cylindrical member 1.06 inches in diameter. This second cover is downwardly inclined at its circumference toward the center to insure complete drainage of the mercury used for charging the calorimeter. All surfaces are nickel plated. The proportions of nickel and of steel are fixed by the manufacturer so as to render it possible to calculate the displacement of the apparatus in water. The calorimeter having been completely filled with mercury is inverted in this liquid in the manner of a test tube. The explosive mixture is then introduced, being fed from a bell in which it has previously been prepared. A rubber tube connects the bell with the instrument. The gas is forced from the bell to the calorimeter by the pressure in the bell. The conical form of the bottom causes the calorimeter to be emptied rapidly and to be refilled completely with explosive gas at a pressure slightly above that of the atmosphere. Equilibrium is re-established by manipulating the valve, during a very short interval, so as to permit the excess gas to escape. During this operation the calorimeter must be maintained in the vertical position shown in the diagram. The atmospheric pressure is read off to one-tenth of a millimeter (0.003936 inches) on a barometer. The temperature of the gas may be taken to be that of the mercury-vessel.