Air Conditioning (Sort of!)
Benjamin Franklin did not "invent" air-conditioning in the strict sense. The first patent for a working system to cool air was issued in 1906, over a century after Franklin's death, to Willis Carrier, an American engineer working in Buffalo, NY. Carrier went on to further refine his invention. In 1933, the company he founded, the Carrier Air Conditioning Company of America, developed major innovations, allowing for practical commerical use of the technology. But like all scientists, Carrier saw far by standing on the shoulders of giants, one of whom was Ben Franklin.
Franklin was not only a great inventor and scientist, he was so firmly committed to the development of science that he never filed for a single patent, not even for the Franklin Stove and bifocals, items he not only invented but constructed and used. In contrast, Franklin never built a working air conditioner, but he did develop and popularize some of the science on which air-conditioning is based.
In a letter to John Lining, dated June 17, 1758, Franklin describes the principle of evaporative cooling, on which modern air-conditioning is based.
I mentioned the experiment for cooling bodies by evaporation, and that I had, by repeatedly wetting the thermometer with common spirits, brought the mercury down five or six degrees. Being lately at Cambridge, and mentioning this in conversation with Dr. Hadley, professor of chemistry there, he proposed repeating the experiments with ether, instead of common spirits, as the ether is much quicker in evaporation. We accordingly went to his chamber, where he had both ether and a thermometer. By dipping first the ball of the thermometer into the ether, it appeared that the ether was precisely of the same temperament with the thermometer, which stood then at 65; for it made no alteration in the height of the little column of mercury. But when the thermometer was taken out of the ether, and the ether with which the ball was wet, began to evaporate, the mercury sunk several degrees. The wetting was then repeated by a feather that had been dipped into the ether, when the mercury sunk still lower. We continued this operation, one of us wetting the ball, and another of the company blowing on it with the bellows, to quicken the evaporation, the mercury sinking all the time, till it came down to 7, which is 25 degrees below the freezing point, when we left off.
In further evidence of his commitment to science not for profit or personal glory, Franklin gives some credit where credit is due, and makes clear that he himself owes a debt to earlier (and non-European) scientists:
It is but within these few years, that the European philosophers seem to have known this power in nature, of cooling bodies by evaporation. But in the east they have long been acquainted with it.
Franklin was not the first person to note the phenomenon of evaporative cooling, nor did he solve the many technical problems to develop a practical air-cooling system, but Carrier and the air-conditioning finally developed in the 20th century owe a debt to Franklin, not only for the experiments he performed and published but for his tireless and selfless commitment to collecting and distributing knowledge.