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The bulbous shapes in the drawings shown here are glass tubes with the air evacuated, leaving a vacuum inside. A metal filament on one end of the glass tube was known to emit "beta rays" when heated. Left to their own devices, the beta rays traveled straight across the tube. Thomson first showed that the beta rays would follow a curved path in a magnetic field, and then that he could deflect them using an electric field. The plates in Experiment 2 generate an electric field between them, which directs the beam of beta rays downward. Thompson also used a similar apparatus to measure the mass-to-charge ratio of the beta rays, which we now know are electrons. (Unit: 1)