The Flat Spiral: Tesla's 1897 Coil Transformer Patent (U.S. 593,138)

2 minute read. Published October 17, 2026.

In short: U.S. Patent No. 593,138, Electrical Transformer, was granted to Nikola Tesla in 1897. It describes a transformer coil wound as a flat spiral, designed to produce very high voltages while keeping the windings safely insulated from each other. It's a refinement of the high-voltage resonant transformers now widely known as Tesla coils.

Tesla's 1897 flat spiral transformer patent drawing printed in amber on a black T-shirt — FIG. 1 from U.S. Patent 593,138, restored and printed in amber on black.

What is a Tesla coil?

A Tesla coil is a resonant transformer that turns ordinary electrical power into extremely high voltages at high frequency, the kind that throw long, crackling sparks through the air. Tesla developed these coils in the 1890s, and they became the engine of his experiments with lighting and wireless transmission.

Why a flat spiral?

At very high voltages, the biggest danger is the insulation failing between parts of the coil that sit at very different voltages. Winding the coil as a flat spiral changes how the voltage is distributed across the turns, which Tesla used to keep high-voltage sections well separated and reduce the risk of breakdown.

Why does it matter?

Without coils that could safely reach enormous voltages, none of Tesla's wireless ambitions, from the upper-air system to Wardenclyffe Tower, would have been possible. This patent is part of the machinery underneath the whole story.

What does the drawing show?

The drawing shows the spiral winding itself, a dense, hypnotic coil that reads almost like sacred geometry. It's one of the most visually striking drawings in the collection.