Chances are you've been through a few revolving doors in your lifetime. They're very common in large buildings and other places with a lot of foot traffic. You may have seen them at an office building or a shopping mall.
Revolving doors consist of three or four doors — often called wings or leaves — that are spaced an equal distance from each other around a center shaft. They rotate in a circle inside of a cylinder. The cylinder can be small, with just enough space for one person between wings, or it can be large enough to allow several people or even strollers or luggage racks to fit between wings.
The revolving door was invented in 1888 by Theophilus van Kannel from Philadelphia, Pennsylvania. An old urban legend holds that revolving doors were invented to prevent horses from entering buildings. Although that's funny, it's not true.
Revolving doors were invented because they have many advantages compared to traditional hinged doors. For example, revolving doors are more energy efficient than regular doors. When you open a regular door, a large amount of air can escape.
Revolving doors also allow more people to enter and exit buildings more quickly. Using a revolving door, people can enter and exit a building at the same time. Regular doors force one person to wait while the other passes through.
Architects like revolving doors because they enhance the entrances of buildings. Not only do they reduce the amount of street noise heard by those inside, they also eliminate the annoying sound of slamming doors.