[Illustration: FIG. 1.--TYPICAL METHODS OF AUTOMATIC GENERATION
CONTROLLED BY BELL GASHOLDER.]
The devices employed to render a generator automatic which depend upon
pressure (B) are of three main varieties: (B^1) the water-level in the
decomposing chamber may be depressed by the pressure therein until its
surface falls below a stationary mass of carbide; (B^2) the level in a
water-store tank may be depressed until it falls below the mouth of a
pipe leading to the carbide vessel; (B^3) the current of water passing
down a pipe to the decomposing chamber may be interrupted by the action
of a pressure superior to the force of gravitation. These arrangements
are indicated roughly in Fig. 2. In B^1, D is a hollow cylinder closed at
all points except at the cock G and the hole E, which are always below
the level of the water in the annulus F, the latter being open to the air
at its top. D is rigidly fastened to the outer vessel F so that it cannot
move vertically, and the carbide cage is rigidly fastened to D. Normally
the water-levels are at _l_, and the liquid has access to the
carbide through perforations in the basket. Acetylene is thus produced;
but if G is shut, the gas is unable to escape, and so it presses
downwards upon the water until the liquid falls in D to the dotted line
_l"_, rising in F to the dotted line _l'_.
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