J.J. Thomson's experiment with cathode ray tubes demonstrated that the beam of particles, now known as electrons, carried negative charges.

Here's why:

* Deflection in Electric Fields: The beam of particles deflected towards the positive plate of an electric field, indicating that the particles themselves were negatively charged.

* Deflection in Magnetic Fields: The beam of particles also deflected in a magnetic field, further confirming the negative charge of the particles.

Thomson's experiment was a crucial breakthrough in understanding the nature of electricity and the structure of atoms.