Patent drawing - System of electric lighting - Nikola Tesla US patent 454622 fig1 Public domain image

Diagram of single wire high frequency lighting system, fig. 1 from Nikola Tesla's US patent no. 454622 System of electric lighting granted June 23, 1891. This is important because it contains the first appearance of his Tesla coil circuit, a resonant transformer which generates high voltage high frequency alternating current (AC). The Tesla coil circuit is in the upper left consisting of a AC generator (G) powering a step-up transformer (P,S) which charges a capacitor (C) which discharges through the spark gap (a) and the primary (P')of the resonant transformer, creating high frequency oscillating currents. The secondary of the transformer (S') produces high voltage pulses.
In Tesla's lighting system the high voltage pulses power prototype single electrode fluorescent light bulbs. These consist of a parially evacuated glass bulb with a single electrode. The high potential ionizes the gas producing a glow discharge The other end of the coil is attached to an electrode in the wall of the room (W)

Alterations to image: cropped out part of wall (W) to make image more compact