A
displacement holder, accordingly, may be used either to store a varying
quantity of gas, or to give a steady pressure just above or just below a
certain desired figure; but it will not serve both purposes. If it is
employed as a holder, it in useless as a governor or pressure regulator;
if it is used as a pressure regulator, it can only hold a certain fixed
volume of gas. The rising holder, which is shown at A^1 in Fig. 1
(neglecting the pin X, &c.) serves both purposes simultaneously; whether
nearly full or nearly empty, it gives a constant pressure--a pressure
solely dependent upon its effective weight, which may be increased by
loading its crown or decreased by supporting it on counterpoises to any
extent that may be required. As the bell of a rising holder moves, it
must be provided with suitable guides to keep its path vertical; these
guides being arranged symmetrically around its circumference and carried
by the tank walls. A fixed control rod attached to the tank over which a
tube fastened to the bell slides telescope-fashion is sometimes adopted;
but such an arrangement is in many respects less admirable than the
former.
Two other devices intended to give automatic working, which are scarcely
capable of classification among their peers, may be diagrammatically
shown in Fig.
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