Final answer:
The type of beam used in this experiment is a cathode ray.
Step-by-step explanation:
The beam used in the famous scientific experiment described is called the cathode ray. In this experiment, a cathode-ray tube is used to produce a narrow beam of electrons that pass through charging plates connected to a high-voltage power supply. The beam is bent when the magnet is on due to the opposing forces of the electric and magnetic fields on the electrons.