In 1904, John Ambrose Fleming invented the thermionic diode, the first kind of vacuum tube. This used a hot cathode that caused an electric current to flow in a vacuum. This idea was quickly applied to X-ray tubes, and hence heated-cathode X-ray tubes, called "Coolidge tubes", completely replaced the troublesome cold cathode tubes by about 1920.
In 1904, John Ambrose Fleming invented the thermionic diode the first kin.d of vacuum tube. This used a hot cath,ode that caused an electric current to flow in a vacuum This idea was quickly applied to X-ray tubes, and hence heated-cathode X-ray tubes, call,ed "Coolidge tubes" completely replaced the troublesome cold cathode tubes by about 1920.