Vacuum tubes are devices that regulate or amplify the flow of electricity. Vacuum tubes were widely used in early and mid-twentieth-century telephones, radios, televisions, and computers. They have since been replaced in most electronic products by solid-state semiconductor devices, such as transistors, which are smaller, more durable, and more efficient. Invented in 1904 by John Ambrose Fleming, ordinary vacuum tubes exploit a phenomenon called thermionic emission, or the “Edison effect.” Thermionic emission is a property of light bulbs that causes a hot filament to throw off electrons. Vacuum tubes consist of sealed glass tubes with electrodes or filaments at either end. At one end of the tube is the cathode. The other end is called the anode (or “plate”). When a voltage is applied to the cathode and anode of a vacuum tube, the cathode becomes negatively charged and the anode becomes positively charged. This creates an electrical field between the cathode and anode, which causes the electrons emitted by the cathode to be attracted towards the anode. As the electrons move towards the anode, they pass through a vacuum, which means there are no other particles present to collide with them or absorb their energy. This allows the electrons to maintain their energy and speed as they travel through the tube. When the electrons reach the anode, they are collected, which causes the anode to become more negatively charged.
The presence and absence of electricity turns the tube “on” and “off.” When the voltage is high enough, the electrons that are emitted turn the device “on.” When voltage declines it turns “off.” Digital computing depends on the power of such binary functioning. Vacuum tube switching is much faster that mechanical or electromechanical switching. The Atanasoff-Berry Computer (ABC), successfully tested in 1942, is the first electronic digital computer to use vacuum tubes. The ABC contained about 300 vacuum tubes. The Electronic Numerical Integrator and Computer (ENIAC) is the first programmable, electronic, general-purpose digital computer to utilize vacuum tube technology. ENIAC, which was completed in 1945, contained more than 18,000 vacuum tubes. Other early computers that relied on vacuum tubes include the National Bureau of Standards’ SWAC (1950), the Soviet MESM (1951), and the AN/FSQ-7 built for NORAD’s Semi-Automatic Ground Environment (SAGE) system (1958). The AN/FSQ-7, with 49,000 vacuum tubes, was the largest vacuum-tube computer ever built.