You're likely thinking of J.J. Thomson and his famous experiment with cathode rays. He didn't call it a "J hypothesis," but his observations led to a groundbreaking conclusion.

Here's what happened:

* Thomson's experiment: Thomson used a cathode ray tube, which emitted a beam of particles from a heated cathode. He then placed electrically charged plates on either side of the beam.

* Observations: He observed that the beam bent towards the positively charged plate and away from the negatively charged plate. This meant the particles in the beam carried a negative charge.

* Conclusion: Thomson concluded that cathode rays are composed of negatively charged particles, which he named electrons. This was a monumental discovery, as it helped establish the existence of subatomic particles.

Key Takeaways:

* Thomson's experiment didn't have a specific "J hypothesis."

* His observations led to the discovery of the electron.

* This was a crucial step in understanding the structure of atoms.

Let me know if you have any more questions!