The X-ray machine was voted by the Science Museum in London. The poll was made to celebrate the 100 years of the Museum.
Almost 50,000 people voted in the museum or online. There were 10 discoveries to choose from.
The X-ray machine was discovered in 1985. It revolutionized how doctors detected diseases and injuries.
The X-ray machine gathered 9581 votes, almost one fifth of the total votes. Other candidates that gathered a good number of votes were penicillin and the DNA double helix structure.
Katie Maggs, associate curator of medicine at the Science Museum, told CNN that she was "pleasantly surprised" with the results, saying she "wondered whether the therapeutic benefit of penicillin might just edge in front -- or perhaps the Apollo 10 capsule as visitors find space travel so inspirational as the ultimate test of technology."
