50 The Nature of Heat
On one visit, Rumford recognized that great quantities of heat flowed into the air and
water from those cannon barrels. At that time scientists believed that, as a substance grew
hotter, more caloric squeezed into it. Eventually caloric overflowed and spilled out in all directions to heat whatever it touched.
Rumford wondered how so much caloric (heat) could pour out of the metal of one cannon barrel—especially since the cannon barrels felt cold when the drilling started.
Rumford decided to find out how much caloric was in each barrel and where that caloric was stored. He fashioned a long trough to catch all the water pouring out of a cannon barrel while it was being drilled so he could measure its increase in temperature.
He also directed that extra hoses be sprayed on the drilling to prevent the formation of
steam. Rumford didn’t want any caloric escaping as steam he couldn’t capture and measure.
Drilling began with a great screeching. Hoses sprayed water onto the drill bits. The
metal began to glow. A torrent of heated water eight inches deep tumbled down the narrow,
foot-wide trough and past the Count and his thermometers.
Rumford was thrilled. More caloric flowed out of that cannon barrel than he could
have imagined in even his wildest dreams. And still the hot water continued to flow past
him, all of it heated to more than 50 degrees celsius.
Eventually the Count’s face soured. Something was very wrong. The metal cannon
barrel had already lost more than enough heat (caloric) to turn it into a bubbling pool of liquid metal many thousands of degrees hot. It seemed impossible for so much caloric to have
existed in the metal.
Count Rumford watched the borers go back to work and realized that what he saw was
motion. As drill bits ground against the cannon’s metal, their motion as they crashed against
the surface of the metal must create heat. Movement was being converted into heat!
Today we call it friction, and know it is one of the primary sources of heat. But in 1790,
no one believed Count Rumford’s new theory of friction heat, and they held onto the notion
of caloric for another 50 years.
Fun Facts: Friction with air molecules is what burns up meteors as they
plunge into the atmosphere. That same friction forced NASA to line the
bottom of every space shuttle with hundreds of heat-resistant ceramic
tiles. Failure of one of those tiles led to the explosion of the Columbia in
2004.
More to Explore
Brown, Sanborn. Benjamin Thompson, Count Rumford. Boston: MIT Press, 1996.
———. Collected Works of Count Rumford. Cambridge, MA: Harvard University
Press, 1994.
———. Count Rumford: Physicist Extraordinary. Westport, CT: Greenwood, 1995.
Ellis, George. Memoir of Benjamin Thompson, Count Rumford. Somerset, PA: New
Library Press, 2003.
van den Berg, J. H. Two Principal Laws of Thermodynamics: A Cultural and Historical Exploration. Pittsburgh: Duquesne University Press, 2004.