Advances in technology in the 20th century resulted in changes at home and in the workplace. In 1900, less than 10% of the nation was electrified; now virtually every home in the United States is wired (Figure 2-11).³² Most of us give little thought to the vast array of electrical appliances that surround us.

As workers left farms to move to cities, transportation systems developed to get them to work and home again. Advances in highway construction in turn fueled the automotive industry. In 1900, one-fourth of US households had a horse, and many in urban areas relied on trolleys and trams to get to work and market. Today, more than 90% of US households own at least one car (Figure 2-12). Improvements in refrigeration put a refrigerator in virtually every home, and the ability to ship food across the country made it possible to keep those refrigerators stocked. The increasing speed, safety, and reliability of aircraft spawned yet another global industry that spans commercial airline service and overnight package delivery.

At the beginning of the 20th century slightly more than 1 million telephones were in use in the United States. The dramatic increase in telephone calls per capita over the following decades was made possible by advances in cable bundling, fiber optics, touch-tone dialing, and cordless communication (Figure 2-13). Cellular-telephone technology and voice-over-Internet protocols have added even more communication options. At the beginning of the 21st century, there were more than 300 million telephone communication devices and cellular telephone lines in the United States.

Radio and television revolutionized the mass media, but the Internet has provided altogether new ways of communicating. Interoperability between systems makes it possible to use one device to communicate by telephone, over the Internet, in pictures, in voice, and in text. The “persistent presence” that those devices make possible and the eventual widespread availability of wireless and broadband services will spawn another revolution in communication. At the same time, new R&D will be needed to