Development of the Self-Regulating Oven
Development of the Self-Regulating Oven
At the start of the seventeenth century, there was no way to measure heat. Although it was known that air expanded as it was heated, and compressed as it was cooled, no one had thought to assign numbers to the degrees of hot and cold. Cornelis Drebbel (1572-1633) was one of a small group of European practical and learned men who worked on developing air thermometers that included numerical scales. Their inventions and innovations illustrate the seventeenth-century trend toward quantifying natural phenomena. But Drebbel's greatest invention was the thermostat.
Drebbel began his career in Holland as an engraver, but turned to mechanical invention in 1598. He had a special interest in how temperature and pressure cause a volume of air to vary. For example, Drebbel observed that heating air and water causes them to expand, whereas cooling compresses them. He described experiments in which he hung an empty glass laboratory vessel, called a retort, with its mouth in a container of water and its bulb toward a flame. As the air in the glass warmed, air bubbled out of the mouth of the retort into the water. But removing the flame from the fire cooled the air and compressed it, drawing water up into the glass.
Drebbel first applied the results of his observations to a kind of perpetual motion machine that he called the perpetuum mobile. The perpetuum mobile was an elaborate toy that Drebbel delighted in showing off to the Emperor Rudolf II (1552-1612) in Prague in 1610. Drebbel divided his working life between England and continental Europe, and his initial fame was probably due to this invention.
Basically, the perpetuum mobile consisted of a glass bulb filled with air and connected with a glass spiral that contained a small amount of liquid. Changes in temperature caused the air in the bulb to increase by day and decrease during the night. These variations caused the liquid in the spiral to move back and forth in a continuous motion, mimicking the flow of the tides. To many, the motions of the liquid in the tube were mysterious; but other of Drebbel's contemporaries realized that what he had built was an air thermometer.
Drebbel later used the principle of the perpetuum mobile in constructing his most notable invention: an oven with an automatic temperature-regulating device that we now call a thermostat. So that he could hatch duck and chicken eggs all year round, Drebbel built an apparatus consisting of a closed pool of alcohol connected to a tube containing mercury. As the temperature rose in the oven, the alcohol expanded, and pushed the mercury up the tube, which opened a valve that admitted cold air. The air then cooled the oven, and as the temperature fell, the volume of alcohol decreased. This caused the mercury column to drop and thereby shut the valve, so that the heat of the oven would again begin to raise the temperature.
Drebbel wrote little, and left no records concerning his thermostatic furnaces. What we know of his work, we know from his contemporaries. After Drebbel's death, the executors of his estate filed an English patent on his furnaces on behalf of his descendants. His inventions did not become widely known until years later. At a meeting of the Royal Society in 1662, in response to a problem proposed by Sir Robert Moray, a member suggested Drebbel's method of regulating a furnace using a mercury thermometer. But the proceedings in which Moray's remarks are quoted provide no technical details. Drebbel's incubators for hatching eggs were mentioned in the Royal Society in 1668. In fact it is Drebbel's son-in-law Johan Sibertus Kuffler (1595-1677) that we have to thank for building several of Drebbel's furnaces and supplying technical information about the temperature regulators.
Drebbel was not alone in proposing ideas for keeping heat at a constant degree. In 1677 the physicist Robert Hooke (1635-1703) described how several oil lamps could be made to burn evenly. And in 1680, Johann Joachim Becher (1635-1682) claimed to have invented temperature regulators. But Becher failed to supply technical details and illustrations.
Although the idea of temperature regulation remained undeveloped in England for the next century, a nobleman from Lyon named Balthasar de Monconys (1611-1665) had visited researchers and learned of Drebbel's thermostatic furnace in conversation with them. Johan Sibertus Kuffler had even shown it to him. Monconys published his observations in a book that appeared in several editions between 1677 and 1697. Thus, though neglected in England, the temperature regulator became common knowledge in continental Europe.
The thermostatic principle of Drebbel's self-regulating furnaces applies to self-regulation of all kinds, in engineering and industry, as well as biological systems such as body temperature. We use the term "feedback control" to describe the mechanisms that drive these systems. Drebbel's thermostatic furnace has been called the first feedback system invented since antiquity.
Feedback control functions to make unstable systems stable. It's what allows quantities such as pressure, temperature, velocity, and thickness to be maintained at a constant level. Feedback control systems have three components: they measure output, compare the output with a desired value, and then make corrections to achieve that value. For example, in a steel rolling mill a feedback mechanism controls the thickness of the billets, in a robot it controls the position of the end of the robot's arm, and in a tank it controls the pressure of the contents of the tank.
As you produce a product, the accumulation of the product slows or stops further production. When something is being heated in the oven, as the temperature of the oven increases, the process of heating decreases. A modern room thermostat has a sensing element that responds to the air temperature in the room, and a control element that regulates the heating process by making a correction whenever the air deviates from the desired temperature. In Drebbel's time, the oven was going constantly. For him, the feedback mechanism didn't reduce the production of heat, as it does in a modern thermostat where the furnace goes off. Instead, for Drebbel, as the heat increased, the feedback mechanism dissipated the heat by applying cold air. Drebbel applied a principle that we realize is a much more significant one than just hatching eggs. It led to the self-control of mechanical devices.
The question persists whether Drebbel can also claim credit for having independently invented the air thermometer. His perpetual motion machine is usually interpreted as one, although Drebbel did not call it that. Certainly, contemporary accounts appear to indicate that he was widely thought to have invented it. A letter written to the philosopher and astronomer Galileo Galilei (1564-1642) mentions that the machine's tube was marked with equidistant diagonal lines. But although it is clear that Drebbel understood the principles involved and could have constructed a thermometer if he had wanted to, the actual inventor remains unknown. Along with Drebbel, the other likely candidates are Galileo; Santorio Santorre (1561-1611), a professor of medicine at Padua in Italy; and the Welsh mystic Robert Fludd (1574-1637).
Drebbel lived in troubled times. The Reformation, a religious revolution that began in 1517, had raised questions that were still not resolved in the early 1600s. Artists and inventors depended for their survival on the favor of kings, and they were often caught up in a web of religious and political machinations. Drebbel himself was arrested and released several times in various countries. The established order of science was still governed by the Aristotelian view of the universe, and by rigidly held ideas about the constitution of matter that had no basis in the observable world.
In his inventions, Drebbel displayed understanding of established scientific principles. Moreover, his research in chemistry and physics may well have contained significant discoveries. But he was also a showman, and sometimes mixed his results with magic to exhibit to an admiring public. What science there was would have been hard to discern behind the hocuspocus. What he was, through and through, was an inventor who had little concern for publicizing his inventions.
Although no one disputes Drebbel's invention of the temperature regulator, he did not himself document his invention, nor did he attempt to produce it on any scale. It was others who in the eighteenth century turned his improvised laboratory device into a carefully designed practical appliance. It is one of the mysteries of the history of technology why some inventions appear and fail to develop, even when the science and know-how to develop them are at hand. One argument, aside from Drebbel's own peculiar methods, is that interest in feedback mechanisms had to await inventions that required them, such as the steam engine or float regulators for domestic water supplies.
Mystery notwithstanding, the importance of the idea of feedback control from the eighteenth century forward cannot be underestimated. Applied to biology, it is known as homeostasis, and refers to the ability of organisms to remain stable while adjusting to conditions they need to survive. It was also adopted in fields beyond engineering, for example, in Adam Smith's (1723-1790) economic postulate of laissez-faire, by which, given the right conditions, economies will automatically swing into equilibrium.
Mayr, Otto. The Origins of Feedback Control. Cambridge, MA: MIT Press, 1970.
Middleton, W. E. Knowles. A History of the Thermometer and Its Use in Meteorology. Baltimore: Johns Hopkins Press, 1966.
Tierie, G. Cornelis Drebbel (1572-1633). Amsterdam: H. J. Paris, 1932.