20th Century Technology

Air Conditioning
Sticky, hot weather became more manageable after Willis Carrier's invention of the air conditioner in 1902. By compressing fluid to produce a cooling effect (a 19th century discovery), new air conditioners allowed workers to labor in greater comfort and kept homes cool. By the 1950s, air conditioners provided relief in cars and trucks. But all this comfort has come at a cost: The Earth's ozone layer has thinned due to the chlorofluorocarbon gases used until recently in refrigerators and air conditioners.

• The Voting Machines of America