The hydraulic crane has a great advantage in possessing an almost ideal brake, for by simply throttling the exhaust from the lifting cylinder the speed of descent can be regulated within very wide limits and with perfect safety.
In the limiting case of a long fine tube, the bore of which varies in such a manner that U is constant, the state of the substance along a line of flow may be represented by the line of constant total heat, d(E+pv) = o; but in the case of a porous plug or small throttling aperture, the steps of the process cannot be followed, though the final state is the same.
The ideal method of determining by direct experiment the relation between the total heat and the specific heat of a vapour is that of Joule and Thomson, which is more commonly known in connexion with steam as the method of the throttling calorimeter.
If steam or vapour is " wire-drawn " or expanded through a porous plug or throttling aperture without external loss or gain of heat, the total heat (E+pv) remains constant (Thermodynamics, § I I), provided that the experiment is arranged so that the kinetic energy of flow is the same on either side of the throttle.
- Throttling Calorimeter Method.
Callendar's experiments on the cooling effect for steam by the throttling calorimeter method gave n =3-33 and c =26.3 c.c. at 100° C. Grindley's experiments gave nearly the same average value of Q over his experimental range, but a rather larger value for n, namely, 3.8.
The value of c is determined by the throttling experiments, so that all the coefficients in the formula with the exception of A are determined independently of any observations of the saturationpressure itself.