The First Vacuum Tube

Sunday, July 19, 2009

William Crookes (1832-1919). This cartoon by Spy, which appeared in the magazine Vanity Fair in 1903, shows Crookes holding one of the glass tubes named after him. Although this elicited ridicule, William Crooke's research nonetheless did reveal the dramatic way that the light effects changed as the pressure dropped. As he reduced the pressure still further, the lights around the "cathode" - the point of entry of the electric current - disappeared, leaving a dark space with the luminous glow reappearing, displaced along the tube. Finally he removed effectively all of the gas, or as much as his most powerful vacuum pumps were able, and increased the voltage to a maximum. With this, the nature of the discharge changed entirely: the aurora disappeared to be replaced by pencil-thin beams emerging in direct lines from the cathode. These were known as "cathode rays."