The rise and fall of this bell is often used to control the
feed mechanism because this movement indicates positively whether enough
gas has been made or that more is required. As the bell lowers it sets the
feed mechanism in motion, and when the gas passing into the holder has
raised the bell a sufficient distance, the movement causes the feed
mechanism to stop the fall of carbide into the water. In practice, the
movement of this part of the holder is held within very narrow limits.
_Gas Holders._--No matter how close the adjustment of the feeding
device, there will always be a slight amount of gas made after the fall of
carbide is stopped, this being caused by the evolution of gas from the
carbide with which water is already in contact. This action is called
"after generation" and the gas holder in any type of generator must
provide sufficient capacity to accommodate this excess gas. As a general
rule the water to carbide generator requires a larger gas holder than the
carbide to water type because of the greater amount of carbide being acted
upon by the water at any one time, also because the surface of carbide
presented to the moist air within the generating chamber is greater with
this type.
_Freezing._--Because of the rather large body of water contained in
any type of generator, there is always danger of its freezing and
rendering the device inoperative unless placed in a temperature above the
freezing point of the water.
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