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The purpose of the aquadag coating on the inside of a cathode ray tube is to

  • A.focus the beam by means of the charge it accumulates during operation
  • B.prevent the glass envelope from being punctured by the electron beam
  • C.screen the deflection plates from external magnetic fields
  • D.collect the secondary electrons emitted by the screen

Answer: D

Electrons striking the phosphor knock out secondary electrons, and without a return path the screen would charge negatively and repel the beam. The conducting graphite coating, held at the anode potential, collects them and completes the circuit.

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