English: Drawing by Serbian American inventor Nikola Tesla showing stages in his evolution of the high frequency resonant transformer used in his Tesla coil. He developed the Tesla transformer around 1891-1892 although this drawing was published in 1899. He describes the steps in the accompanying article:

(1) Closed-core transformers used at low frequencies

(2-7) Rearranging windings for lower losses

(8) Removed iron core

(9) Partial core

(10-11) Final conical Tesla transformer

(12-13) Tesla coil circuits

The high Q RF transformer designs invented by Tesla have been used in radio transmitters to the present day.

Caption: Fig. 2 TABLE OF DIAGRAMS ILLUSTRATING EVOLUTION OF A HIGH-TENSION TRANSFORMER ENABLING THE PRODUCTION OF ELECTRO-MOTIVE FORCES OF MANY MILLIONS OF VOLTS