Thermodynamics

Tuesday, November 11, 2025 09:05PM

Franklin noted a principle of refrigeration by observing that on a very hot day, he stayed cooler in a wet shirt in a breeze than he did in a dry one. To understand this phenomenon more clearly, he conducted experiments. In 1758 on a warm day in Cambridge, England, he and fellow scientist John Hadley experimented by continually wetting the ball of a mercury thermometer with ether and using bellows to evaporate the ether.[189] With each subsequent evaporation, the thermometer read a lower temperature, eventually reaching 7 °F (−14 °C). Another thermometer showed that the room temperature was constant at 65 °F (18 °C). In his letter Cooling by Evaporation, Franklin noted that, "One may see the possibility of freezing a man to death on a warm summer's day."[190]

In 1761, Franklin wrote a letter to Mary Stevenson describing his experiments on the relationship between color and heat absorption.[191] He found that darker color clothes got hotter when exposed to sunlight than lighter color clothes, an early demonstration of black body thermal radiation. One experiment he performed consisted of placing square pieces of cloth of various color out in the snow on a sunny day. He waited some time and then measured that the black pieces sank furthest into the snow of all the colors, indicating that they got the hottest and melted the most snow.

According to Michael Faraday, Franklin's experiments on the non-conduction of ice are worth mentioning, although the law of the general effect of liquefaction on electrolytes is not attributed to Franklin.[192] However, as reported in 1836 by Franklin's great-grandson Alexander Dallas Bache of the University of Pennsylvania, the law of the effect of heat on the conduction of bodies otherwise non-conductors, for example, glass, could be attributed to Franklin. Franklin wrote, "... A certain quantity of heat will make some bodies good conductors, that will not otherwise conduct ..." and again, "... And water, though naturally a good conductor, will not conduct well when frozen into ice."[193]

Tuesday, November 11, 2025 09:06PM