Mechanical types of adjustable-speed drives and other electrical types continued to be used and new types developed after the Ward Leonard system was introduced. Electron tube types of DC motor controls began to be developed in the 1920s but electronic controls didn’t seriously begin to displace the Ward Leonard system until thyristor controlled drives were developed in the late 1960s. By the mid-1970s, Ward Leonard drives were rapidly becoming obsolete, but the replacement of existing Ward Leonard drives has continued past the end of the 20th century.