Maxwell
Is light itself electric and magnetic?
That time, that place
Maxwell's work was not a new experiment. It was binding the known experimental laws of electricity and magnetism into one system of equations — and translating Faraday's imagined lines of force into mathematics.
Why this question
In binding them he found a problem: joined as they stood, the laws allowed situations where charge is not conserved. Charge would simply vanish somewhere.
No experiment was wrong. The equations simply did not fit together.
What was found
He added a term: a changing electric field produces a magnetic field. No experiment supported it. It was there to make the equations consistent.
With the term in place, the equations gave an unexpected solution — a wave that sustains itself, the electric field generating the magnetic and the magnetic regenerating the electric as it travels.
He computed the speed of that wave. It came out equal to the already-measured speed of light.
Light was an electromagnetic wave. And there had to be others at other wavelengths — light no eye can see.
Electricity, magnetism, and light were separate phenomena
The wave speed from the equations matching the measured speed of light
Twenty-three years later Hertz generated radio waves in the laboratory. Special relativity also comes out of these equations