On June 27, 1914, the last pole was erected at Wendover, Utah, on the Nevada/Utah state line, and was topped with the American flag. Commercial service was started on Jan. 25, 1915. Inset: Stamp from the U.S. Post Office's "Celebrate the Century" series commemorates AT&T's achievement.

Jan. 25, 1915 - If you collect U.S. postage stamps, you might have noticed one issued in February 1998 to commemorate AT&T's 1914 construction of the first transcontinental telephone line.

AT&T began building the nation's original long distance network in 1885. Starting from New York, the network reached Chicago in 1892. But, because an electrical signal weakens as it travels down a wire, that distance was close to the limit for a line built of thick copper. With the 1899 introduction of loading coils, which slow the rate at which a signal weakens, construction proceeded west. By 1911, the network stretched as far as Denver, but had reached the distance limit for loading coils.

In 1908, AT&T President Theodore Vail had made a transcontinental telephone line a major goal, even though he knew the technology to build one did not exist. The next year, Chief Engineer John J. Carty raised the stakes when he announced in San Francisco that AT&T would open a transcontinental line in time for the city's 1915 exposition to mark the completion of the Panama Canal.

But, without a scientific breakthrough, AT&T couldn't make good on that bet. To improve the company's odds, Carty not only hired physicist Dr. Harold Arnold to study the amplification of electrical signals, he also spread the word in the scientific and electrical-engineering community that AT&T would pay handsomely for an electrical amplifying device.

With the development of vacuum tube amplifiers, it was possible to extend the New York-Denver circuit to San Francisco.

On Oct. 30, 1912, independent inventor Dr. Lee de Forest brought the audion, a three-element vacuum tube, to AT&T's engineering department. De Forest's invention provided a small amount of amplification, and then broke down into a bright blue haze. However, Arnold recognized almost immediately that the blue haze was caused by ionization of residual gasses in the tube. If he increased the vacuum, thereby removing most of the residual gasses, the audion would become a practical amplifier. So on Arnold's recommendation, AT&T bought the patent rights from de Forest.

By summer 1913, AT&T had tested high-vacuum tubes on the long distance network. And that fall, the company began constructing the line west from Denver and upgrading the line to the east. On June 27, 1914, AT&T completed the line, erecting the last pole at Wendover, Utah.

Only one problem remained: AT&T had connected the continent before the Panama-Pacific exposition was ready. So the company waited, and on Jan. 25, 1915, opened the line with great fanfare and celebrations in San Francisco and New York.