In 1605, Cornelis Drebbel moved from his native Netherlands to England, where his “magical” inventions amused the court of James I. He was no scholar, but this vulgar mechanic produced one of the first modern feedback systems, a thermostat, for his magical oven:
t was an L-shaped glass tube filled with alcohol that was topped off by mercury. A metal rod floated in the quicksilver. When the heated alcohol expanded, it pushed up the quicksilver, and the rod rose in the tube. The rising rod then pressed on a lever arm, which adjusted the size of a vent at the top of the furnace. As a chimney designer, Drebbel would have known how flue vents control fires. Flames in an oven can be controlled by the size of the aperture above, which determines the draft. Likewise, the fire that heated Drebbel’s furnace was tempered by the size of the vent hole at the top of the oven, which was in turn managed by the rising and falling rod.
The oven comprised three metal boxes that were nested like Russian matryoshka dolls, with a fire below them. The outer box enclosed a chimney-like passage that swept around the other two and exited out a vent at the top. The next box held water, a heat buffer to protect the center box, which contained a sample to be heated. The base of the thermostat, or “feeler,” slipped into the water-filled box. It registered the fire’s heat and responded to changes by automatically adjusting the valve in a way that curtailed or boosted the heat of the fire. What’s more, a person could set the desired temperature, and the oven would oblige.