A set of lightbulbs belonging to Thomas Edison could fetch US $500 000 when it goes under the hammer in London next week. The set contains rare bulbs dating from the 1880s, including several made by Edison and his fierce competitor, Joseph Swan. But the star of the collection is one of Edison's unappreciated inventions: a bulb that may well have been the world's first vacuum tube.

The items were collected to serve as evidence in a trial in which Edison successfully sued the U.S. Electric Light Co. for violating a patent on the design of his lightbulb. Edison, who was famously litigious, collected examples of the designs used by leading lightbulb makers in an effort to show that U.S. Electric Light must have copied his version. At the trial, two of the bulbs were deliberately broken so that the court could study their construction.

The evidence was lost after the trial, apparently after John Howell, Edison's secretary, stored it in an attic. "By rights, it shouldn't even still exist," says Laurence Fisher, a specialist in technical apparatus at Christie's auction house in London, where the sale takes place on 13 December. The bulbs remained untouched for a century, until discovered about a year ago by a descendent of the owner in whose attic they were stored. The bulbs are now offered for sale by a seller who wishes to remain anonymous.

Among the most valuable items in the collection is Edison's vacuum tube. The date of manufacture raises the question of whether Edison should be credited with the invention of the diode—a critical element in the development of electronics.

Edison built the device after becoming curious about the patterns of soot that accumulated on the inside of ordinary bulbs. He became convinced that the carbon was deposited by current flowing outside the filament; in an attempt to catch this current, he added an extra electrode to the design. This extra electrode—today called an anode—did indeed catch the current but only when it was flowing in one direction. That is, it functioned just like a modern diode. The bulb is shown on this page; click on the image to get a close-up of the gap to the anode.

Edison patented the device as the Edison effect lamp, suggesting that it might be useful for governing the current from a dynamo. However, he never pursued the idea. It might have been forgotten had Edison not displayed the bulb at the Philadelphia Exhibition in 1884, where it was spotted by an English engineer, who brought the idea home with him.

It came to the attention of John Fleming, the first professor of electrical engineering at University College, London. Fleming made numerous refinements and improvements to the design, publishing the results of his research in 1889. Yet he, too, could at first find no application for the tube, which he called a thermionic valve.

That changed when Guglielmo Marconi invented the radio, in the 1890s, and hired Fleming as a consultant to improve the design, which could merely detect the presence of radio waves, not measure changes in their amplitude. For that reason, early radios could serve only as wireless telegraphs.

They relied on a "coherer"—a glass tube of iron filings that reacted to an electromagnetic field by becoming magnetized, so that they stuck together, changing their resistance to a current passing through the tube. The coherer was clumsy, even for the purpose of telegraphy, as its filings had to be unstuck by tapping the device after every burst of radio energy.

Fleming at last saw a use for his valve. When he connected it to an aerial in which radio waves had induced electrons to move back and forth, only current flowing in one direction was able to pass through the valve. This design thus converted an alternating signal into a linear one.

Keith Thrower, an historian of the diode, says Fleming's work was critical for two reasons: it paved the way to the amplification of radio signals and it made it possible to receive voice transmissions. Voice is a continuously varying signal and thus beyond the capabilities of a coherer.

But suggestions that Edison missed a trick are unfair, says John Liffen, a curator of communications at the Science Museum, in London. "Nobody could have realized the significance of the thermionic bulb until after the invention of radio," he says. "Edison was simply ahead of his time."

Thrower agrees. "Fleming made many very important changes to the design and applied it to radio," he says. "There's no question in my mind that he should get the credit."

Anybody wanting to bid for the collection next week will need deep pockets. Fisher has estimated the value of the collection at around $500 000 but adds that the dynamic of the auction could push it far higher.