Parmelee Fire Sprinkler

Early factory fire protection was a perforated pipe running along the ceiling with a valve at the end of the building. When a fire broke out, somebody had to notice it, reach the valve, and open it — soaking every floor of the mill in the process.

Parmelee's sprinkler decides for itself, locally. Each head is held shut by a joint of low-melting-point alloy. Heat from a fire directly beneath melts that joint, the head opens, and water falls on that spot alone. No detector, no wiring, no person: the fire operates its own extinguisher.

The elegance is that the sensor, the decision and the valve are one component, and the failure mode is chosen well — a head that fails tends to fail by opening. US Patent 154,076, "Improvement in fire-extinguishers", granted to Henry S. Parmelee of New Haven on 11 August 1874.