One hundred five years ago George A. Darby, an electrical engineer in Birmingham, England, patented the Electrical Heat-Indicator and Fire Alarm.  This device was a heat detector that reacted by an increase in temperature in a room where it was located. 

This ingenious device was ahead of it's time, operating by closing an electrical circuit to sound an alarm if the temperature rose above what was considered as a safe level in 1902.  The contact was made by bridging a gap with a conductor by allowing one plate to fall on another.  The connection of the two plates was caused simply by a block of butter which melted as the temperature increased. This early device subsequently gave way to modern fire alarms and eventually smoke alarms.

The first home smoke detectors were invented by Duane D. Pearsall in 1967.