A cathode ray tube is made of glass with a small amount of some kind of gas in it. It has metal electrodes at each end to pick up an electric current. The electrodes are named "positive” and "negative." What are one of the main uses of this device?

A cathode ray tube produces a beam of electrons, which are charged particles, and lets the beam be observed or examined. Therefore, its main use among these choices is to examine a beam of charged particles. It does not change a proton’s strength or magnetize atoms.

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