Tesla's Discovery around 1930
In the 1930's Tesla announced he could make free electricity from some hidden power of the Universe. He was tardy about how his system worked but it became known as "Tesla's Coil".
I saw a description in a reference in an old book I had as a child.
It was simple: A hydrogen Geisler tube inside an induction coil with a Layden jar across the coil to make a parallel tuned circuit. The power output was taken from a secondary. Power level was controlled by a variable spark gap that shunted the coil to keep a constant power level it was adjusted by moving the breaker contact.
The alternating current was generated by a direct current that was broken by a vibrating reed circuit breaker in series with a source of direct current and the coil. The magnetic force of the coil with current flowing pulled the reed so that is broke the circuit. The reed was tuned to the resonant frequency of the tuned circuit.
One report was that the tube was a vacuum but this probably means low pressure air. Air is also a nuclear fuel as the atomic nuclei in air will occasionally fuse to form heavier elements with the release of energy. Hydrogen was reported as the best fuel but helium and neon were reported inactive
In the arrangement below the energy from spontaneous fusion reactions at room temperature is picked up by the tuned circuit and stored so that as the electrical energy accumulates the feedback from the tuned circuit to the gas causes heating and a higher and higher rate of reaction until useful power can be extracted.
My ideas are derived from this and my own repeats of his experiments when I was about ten.
The knowledge of thermonuclear fusion was not known then and the source of the energy was a complete mystery.
It has taken me forty years to work out the physics and these dangerous "hydrogen engines" have caused fatalities.