Engines of Our Ingenuity
No. 141:
FRANKLIN'S HEAT EXPERIMENT

by John H. Lienhard

Click here for audio of Episode 141.

Today, Benjamin Franklin thinks about theoretical and applied science. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

The French economist Turgot said about his contemporary, Ben Franklin, "He snatched lightning from the skies and the sceptre from tyrants." That's about as close as you can get to putting Franklin in a nutshell. Franklin's breadth and intelligence were dazzling by any measure. His life spelled out the progress of the American Revolution -- from intellectual adventurism in the early 18th century all the way to an established new nation.

When he was only 23 Franklin turned his omnivorous curiosity on heat radiation. He belonged to a scientific club that eventually became the American Philosophical Society. Franklin cooked up an experiment and another member helped him. They laid colored cloth patches -- and a pane of glass -- on the snow one sunny winter day. Then they noted how deeply each melted into the snow. The white cloth hardly sank at all. The darker each patch was, the deeper it sank. The black cloth and the glass pane sank deepest.

Franklin didn't publish these results until much later. By then he'd added a twist: he also focused a burning glass on both white and black paper. The white paper absorbed less heat, so it took longer to catch fire than the dark paper.

Franklin brought out issues that weren't explained until the 20th century. The explanation goes like this: black cloth is black because it absorbs light. White cloth reflects light. Heat radiation is a lot like light, but not entirely so. Some surfaces respond differently to light and heat -- skin, for example. Black skin absorbs light. But it reflects heat just the way white skin does. If it didn't, Nigerians would all have to live in Stockholm. Ben Franklin's pane of glass made a similar point. Light goes right through glass. But it absorbs the heat carried in sunlight. Glass is black to heat radiation.

The young Franklin concluded by saying, "What signifies Philosophy that it does not apply to some Use?" And he went on to say what his tests suggested about dressing for cold and warm climates.

Years later, an aging Franklin watched an early balloon ascent in Paris. The man next to him said, "What good is it?" and Franklin replied, "What good is a newborn baby!" The older Franklin was answering the younger Franklin's question, "What signifies Philosophy that it does not apply to some Use."

Early-20th-century physicists eventually showed us the newborn baby in the young Franklin's experiments. His simple tests exposed behavior that couldn't be explained until we'd completely rewritten the laws of nature. Their value lay far beyond the obvious utility of choosing the right clothes for a sunny day.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Cohen, I. B., Benjamin Franklin's Science. Cambridge, MA: Harvard University Press, 1990. Chapter 9.

This episode has been greatly revised as Episode 1609.



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The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.


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