Final answer:
J.J. Thomson used a cathode ray tube in 1897 to discover electrons by measuring their charge-to-mass ratio and observing their behavior in electric and magnetic fields.
Step-by-step explanation:
The person who used a cathode tube to help discover that electrons exist was J.J. Thomson. In 1897, J.J. Thomson carried out experiments with a cathode ray tube, which led to the groundbreaking discovery of the electron. By observing the deflection of cathode rays in electric and magnetic fields, Thomson deduced that the rays were composed of negatively charged particles, which he called electrons. These observations allowed him to measure the charge-to-mass ratio of these particles, further substantiating their existence and characteristics.
A cathode ray tube consists of two electrodes: the cathode (negative) and the anode (positive). When an electric current is passed through, a beam, known as the cathode ray, emanates from the cathode to the anode. Thomson's experimentation with different cathode materials revealed consistent properties of the cathode rays, indicating these were a fundamental component of atoms: the electron. His work laid the foundation for our modern understanding of atomic structure.