Noun
A vacuum tube in which the exhaustion is carried to a very high degree, with the production of a distinct class of effects; -- so called from W. Crookes who introduced it.
Source: Webster's dictionaryHe built a Crookes tube (later called a "Lenard tube") with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it. Source: Internet
The simplest and cheapest variety of sealed X-ray tube has a stationary anode (the Crookes tube ) and run with ~2 kW of electron beam power. Source: Internet
Taking an X-ray image with early Crookes tube apparatus, late 1800s. Source: Internet