Vacuum tube with metal cross makes shadow with cathode rays.

Vacuum tube with metal cross makes shadow with cathode rays. An evacuated Crooke's discharge tube with a hinged metal cross is used to illustrate that cathode rays travel in straight lines. When a high voltage (about 40 kV pulsating D.C.) from an induction coil is placed across the tube, a beam of electrons is emitted from the cathode, casting a shadow of the cross on the glass envelope. The glass fluoresces green, and in the shadow it remains dark. When the glass is bombarded continuously by cathode rays, the fluorescence grows fainter due to 'fatigue' . If the hinged metal cross is tipped down, the fresh glass that was in shadow glows brighter green. Ref.:Modern College Physics by Harvey White, 6th ed., p. 624-627