Before revision: The glass bulb or link hold in place a pipe cap which acts as a plug to prevent water from flowing, unless the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler head.
After revision: The glass bulb or link hold in place a "pip cap" which acts as a plug to prevent water from flowing, unless the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler head.