Final answer:
J.J. Thomson's experiments with electric and magnetic fields showed that cathode rays carry a negative electric charge.
Explanation:
Cathode rays were shown to be negatively charged by J.J. Thomson through experiments with electric and magnetic fields. When a cathode ray is subjected to these fields, it bends in a manner consistent with a negative charge, deflecting towards the positive electrode. This demonstration led to the understanding that cathode rays carry a negative electric charge.
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