On February 23, 1897, Nikola Tesla received a patent for electrical condensers and coils.

The features of the patent:

1. Improvements of condensers, transformers, self-induction coils, rheostats, and other similar devices

2. Used in areas where currents of high potentials are brought into close proximity

3. Method of excluding gas or air from the dielectric environment of such devices

4. Insulated material rendered fluid by heat

5. Material permeated the interstices of device and held under pressure

6. Material cooled and solidified under pressure.

US577671