What is the operating element of an automatic sprinkler head that can be used along with a heat-sensitive bulb?

The operating element of an automatic sprinkler head designed to activate when heat is applied is the soldered link. This component typically consists of two metal pieces that are held together through a solder that melts at a specific temperature. When the temperature rises due to a fire creating heat, the solder melts, causing the link to separate. This action releases the sprinkler head and allows water to flow, effectively dousing the flames.

Using a heat-sensitive bulb as an alternative mechanism in some sprinkler systems relies on similar principles but acts differently. The bulb contains a liquid that expands with heat to break the glass, triggering water release. However, the soldered link operates purely on the melting of metal rather than the expansion of a liquid.

The other options, such as an electronic switch or thermostatic valve, do not function in the same manner as the soldered link in an automatic sprinkler system. An electronic switch is more common in systems requiring electrical activation, while a thermostatic valve is often associated with controlling temperatures rather than the direct activation needed in fire sprinkler systems. Thus, the soldered link is the appropriate choice, as it specifically pertains to the mechanical response in a typical automatic sprinkler head setup.