When this valve is first opened the carbide runs into the water until
sufficient pressure and volume of gas is generated to raise the bell. This
movement operates the arm attached to the carbide shut off valve and slowly
closes it. A fall of the bell occasioned by gas being withdrawn again opens
the valve and more gas is generated.
_Mechanical Feed._--The previously described methods of feeding
carbide to the water have all been automatic in action and do not depend
on the operator for their proper action.
Some types of large generating plants have a power-driven feed, the power
usually being from some kind of motor other than one operated by a weight,
such as a water motor, for instance. This motor is started and stopped by
the operator when, in his judgment, more gas is wanted or enough has been
generated. This type of machine, often called a "non-automatic generator,"
is suitable for large installations and is attached to a gas holder of
sufficient size to hold a day's supply of acetylene. The generator can then
be operated until a quantity of gas has been made that will fill the large
holder, or gasometer, and then allowed to remain idle for some time.
_Gas Holders._--The commonest type of gas container is that known as a
gasometer. This consists of a circular tank partly filled with water, into
which is lowered another circular tank, inverted, which is made enough
smaller in diameter than the first one so that three-quarters of an inch is
left between them.
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