The correct answer is option 3, James P. Joule. James P. Joule was a physicist from England who is known for his work on the relationship between heat and mechanical work. He conducted numerous experiments to understand the effects of electric current on heating. Through his experiments, Joule discovered the heating effect of electric current, which is now known as Joule`s law.

Option 1, Isaac Newton, was a renowned physicist and mathematician but his work was primarily focused on the laws of motion and gravitation, and he did not investigate the heating effect of electric current.

Option 2, Galileo Galilei, was a physicist and astronomer known for his contributions to the scientific method and his observations of the motion of objects. He did not specifically study the heating effect of electric current.

Option 4, J.J. Thomson, was a physicist who made significant contributions to the understanding of the nature of electric current and the discovery of the electron. However, he did not pioneer the discovery of the heating effect of electric current.

Therefore, the correct answer is option 3, James P. Joule, who was a pioneer in discovering the heating effect of electric current.