Air-conditioning blew into residential use in the late 1940s, eventually giving rise to the ranch house--and the computer revolution. Courtesy Carrier Corporation If man has the intelligence to heat his house in the wintertime, why does he not cool it in the summer?" asked Alexander Graham Bell in 1918. Blame the second law of thermodynamics, which says that heat can't pass from a cooler object to a hotter object. Instead, the hot body will warm up the cool one (see, for example, Casablanca or Some Like It Hot). That makes it hard to bring the indoor temperature down to a comfortable 68 degrees Fahrenheit when outside it's 99 in the shade.
By the nineteenth century, however, a few sweaty visionaries had already found a loophole that enabled them to invent chilling machines. When a substance expands, spreading its heat over more volume, its temperature drops. Similarly, compressing a substance crowds the heat into a smaller space, bringing the temperature up. In a window air conditioner, refrigerants expand into coils, lowering their temperature. Fans blow warm, indoor air over the cold coils, cooling the air and heating the refrigerants, which then pass to a compression unit and are squeezed into a tiny volume. The temperature of the refrigerants shoots up above that of the hot, outdoor air, which can then carry away the excess heat. Got it?
