Ever since Alexander Graham Bell introduced the telephone at Philadelphia's Centennial Exhibition in 1876, telecom companies have promoted the communication device at world's fairs, trade shows, and state fairs, including Northwestern Bell Telephone Co. at the 1957 Minnesota State Fair pictured here. By the late 1950s, with 76 percent of households owning a telephone, and the market therefore maturing, telecom firms began pushing consumers to add more phones in their homes. By 1970, the number of phones per 1,000 people in the United States jumped by almost 60 percent.