This is the lecture that made Nikola Tesla famous. Looking back on this lecture in his 1919 article "The True Wireless," Tesla wrote the following:
In the spring of 1891 I gave my demonstrations with a high frequency machine before the American Institute of Electrical Engineers at Columbia College, which laid the foundation to a new and far more promising departure. Although the laws of electrical resonance were well known at that time and my lamented friend, Dr. John Hopkinson, had even indicated their specific application to an alternator in the Proceedings of the Institute of Electrical Engineers, London, Nov. 13, 1889, nothing had been done towards the practical use of this knowledge and it is probable that those experiments of mine were the first public exhibition with resonant circuits, more particularly of high frequency. While the spontaneous success of my lecture was due to spectacular features, its chief import was in showing that all kinds of devices could be operated thru a single wire without return. This was the initial step in the evolution of my wireless system.
Transformers
AC transformers are devices used to increase or decrease the voltage of alternating current (AC) electricity while maintaining the same frequency. Transformers consists of two coils: the primary coil and the secondary coil.
Primary: This coil is connected to the source of electricity. A varying current is passed though it.
Secondary: This coil is positioned close to or around the primary coil. The secondary coil has a significantly larger number of turns of wire. The primary induces a voltage in the secondary.
The cylinder in between the two coils represents the "core" of the transformer. The core is made out of a material with high magnetic permeability, such as iron or ferrite.
In the last lecture, an electrostatic voltmeter was shown to measure the rate of change of magnetic induction. Transformers use this effect to step up or step down the voltage. The induction coil was the first transformer, but before Tesla's Artificial Illumination lecture it was thought of as a classroom toy rather than a practical device. Tesla's novel approach was to use high-frequency currents to operate the induction coil.
Tesla presented many different illumination effects produced by the induction coil when operated with high-frequency AC.
Electrostatic Lightbulbs
Tesla developed several designs for lightbulbs which operate via these high-frequency electrostatic effects.
We may place a body of some refractory material in a closed, and preferably more or less exhausted, globe, connect it to a source of high, rapidly alternating potential, causing the molecules of the gas to strike it many times a second at enormous speeds, and in this manner, with trillions of invisible hammers, pound it until it gets incandescent.
Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination (1891)
Tesla demonstrated this effect by directly attaching bulbs to the secondary of the induction coil. He could control the brilliance of the bulb by adding a capacitive coating to the outside of the bulb and connecting it to an insulated plate, the size of which he varied to control the brightness.
Tesla even developed lightbulbs where the secondary is included in the stem of the bulb so that they could be operated wirelessly. Shown are two bulbs which utilize one or both terminals of a built-in secondary coil.
Electrostatic Effects
Tesla's recommended these electrostatic illumination effects for practical lighting over ordinary incandescent filament bulbs. Tesla described traditional lightbulb filaments as wasteful.
When Geissler tubes (glass tubes containing rarefied gas) are placed in an alternating electrostatic field, they illuminate.
Electric Transients
Electric transients are produced when an inductor or a capacitor is suddenly discharged.
Most people have heard of AC and DC, but there are two other forms of electricity: impulse current and oscillating current.
Tesla used the discharge of a capacitor to generate his electric transients. The figure shows the current over time as the capacitor discharges.
The impulse manifests as a current spike, due to the rapid rate of change of the electrostatic field in between the capacitor plates as it discharges.
Oscillating current is important because the frequency of the oscillations is far greater than normal AC. This frequency, called the resonant frequency, is determined by the capacitance and inductance of the system. Capacitance and inductance both lower the resonant frequency. A resonant circuit can be "tuned down" by increasing either the inductance or the capacitance, or "tuned up" by decreasing either the inductance or the capacitance.
Here are Steinmetz's comments on the utility of oscillations:
Even going to the limits of peripheral speed, and sacrificing everything for high frequency, a limit is reached in the frequency available by electrodynamic generation.
It becomes of importance, therefore, to investigate whether by the use of the condenser discharge the range of frequencies can be extended…
It appears that the highest frequency of oscillation of appreciable power which can be produced by a condenser discharge reaches billions of cycles per second, thus is enormously higher than the highest frequencies which can be produced by electrodynamic machinery.
At five billion cycles per second, the wavelength is about 6 cm., that is, the frequency only a few octaves lower than the lowest frequencies observed as heat radiation or ultra red light.
Theory and Calculation of Transient Electric Phenomena and Oscillations
These oscillations were valuable to Tesla, because the illumination effects he observed were primarily electrostatic and the "straight electrostatic thrusts" he pursued were stronger at high frequencies. The higher the frequency, the lower the voltage required to produce the illumination effects.
High frequencies are especially wanted, for practical considerations make it desirable to keep down the potential. By the employment of machines, or, generally speaking, of any mechanical apparatus, but low frequencies can be reached; recourse must, therefore, be had to some other means. The discharge of a condenser affords us a means of obtaining frequencies by far higher than are obtainable mechanically, and I have accordingly employed condensers [capacitors] in the experiments to the above end.
Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination (1891)
Tesla produced these capacitor discharges by using a spark gap. The induction coil charges up the capacitor until the voltage across the spark gap is high enough for a spark to leap across the gap. This bridge across the spark gap allows the capacitor to discharge into the receiving circuit in the form of a powerful transient.
We may look upon the arc playing between the knobs as being a source of alternating, or generally speaking, undulating currents, and then we have to deal with the familiar system of a generator of such currents, a circuit connected to it, and a condenser bridging the circuit.
Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination (1891)
Resonant electricity has some remarkable properties that differ from regular AC or DC. The impulse currents light up the light bulbs even when they are shorted out!
Tesla even discusses discharging the capacitor into yet another transformer, as shown. This is a Tesla transformer!
These early insights paved the way for Tesla's further investigation of electrical resonance.