Sparks Explained: Creation, Light, Sound, and Tesla Coil Music

The Royal Institution, with its 200-year history of scientific exploration, hosted an evening dedicated to unraveling the mysteries of sparks, their creation, and their surprising applications, including making music. The presentation was led by Michael Cuts from the demo and heritage teams, and Professor Ricketts, the innovator in residence.

How Sparks Are Made

A spark is essentially electricity breaking through an insulator. Air, despite being invisible, is an excellent insulator. To force electricity through air, a significant voltage is required—approximately 30,000 volts per centimeter. This high voltage "rips" electrons from one end and shoves them to the other, creating an "avalanche" of electrons that forms a low-conductance path of ionized air, allowing the spark to flow.

Static Electricity and Lightning

One common way to generate a spark is through static electricity. Rubbing a plastic rod against fur physically rips electrons away, building up a static charge. This principle is scaled up dramatically in nature to create lightning. Water molecules and ice colliding within clouds steal charges from each other, accumulating massive amounts of static electricity. When this charge becomes too great, it discharges as a lightning bolt, which can involve millions of volts.

Inducing High Voltage from Low Voltage

While 30,000 volts is a substantial amount, sparks can be generated from much lower voltages, such as 12 volts, by utilizing the principle of electromagnetic induction, discovered by Michael Faraday at the Royal Institution.

When current flows through a conductor, it creates a magnetic field. Faraday found that a change in a magnetic field induces a voltage. If a current flowing through a wire is suddenly interrupted, the magnetic field collapses, inducing a very high voltage. This induced voltage can be tens of thousands of volts, enough to create a spark as it seeks to maintain the current flow. This is why sparks are often seen when electrical connections are disconnected, not just connected.

The Induction Ring and Transformer

Faraday's induction ring demonstrated this principle by using two coils of wire. Passing current through one coil (the primary) induces a current in the second coil (the secondary). By having more turns in the secondary coil than the primary, the induced voltage can be amplified. Faraday meticulously insulated his own wires with silk for his original induction ring, which can still be seen at the Royal Institution.

Modern transformers, like those found in black tubes, apply this concept. They consist of two coils wound around an iron core, with a significant difference in the number of turns. To prevent sparks from discharging in the air at very high voltages, these transformers are often filled with oil, which is a better insulator than air. An electrical switch rapidly connects and disconnects the primary coil, continuously inducing high voltage in the secondary. This allows a 12-volt input to generate enough voltage to create a spark across a 5-centimeter gap.

The Role of Vacuum

Air acts as an insulator. By reducing the amount of air, or creating a partial vacuum, the insulation is decreased, making it easier for electricity to flow and create a spark over a larger distance. In a demonstration, an 11-centimeter gap that wouldn't spark in normal air readily produced a continuous stream of electricity in a partial vacuum.

Tesla Coils and Resonance

Nicola Tesla revolutionized high-voltage generation with his Tesla coil, which utilizes resonance. Unlike a simple induction coil (like a "Ruhmkorff machine"), a Tesla coil has two resonant circuits: a primary and a secondary. This resonance allows small electrical "perturbations" to build up enormous voltages, similar to how a small tap can make a tuning fork vibrate intensely or how a pendulum swings with increasing amplitude.

In a Tesla coil, the primary coil acts as an inductor, and the large metallic sphere or toroid at the top acts as a capacitor. This capacitor stores electrical potential energy, and the inductor stores energy in its magnetic field. The rapid oscillation of energy between these components, driven by the primary circuit, creates hundreds of thousands of volts. Tesla himself demonstrated his resonant transformer at the Royal Institution in 1892.

What We See When We See a Spark

While we perceive sparks as visible light, we are not actually seeing the electrons themselves. Electrons are thousands of times smaller than the wavelengths of visible light.

The Cathode Ray Tube Experiment

In a near-perfect vacuum, such as a cathode ray tube (CRT), electrons can be seen to flow by their effect on a phosphorescent screen. When electrons hit the screen, they excite the phosphorus atoms, causing them to emit green light. A Maltese cross placed in the path of the electrons casts a shadow, proving the particulate nature of electrons. However, the electrons themselves remain invisible.

Excitation of Gases

The light we see in sparks and electrical discharges is due to the excitation of gas molecules. When electrons flow through a gas, they collide with the gas atoms, exciting their electrons to higher energy levels. When these excited electrons fall back to their original energy levels, they emit photons of light. The color of this light depends on the type of gas and the energy difference between the electron shells.

Examples include: - Neon: Emits a yellow-orange light. - Argon: Emits a purple light. - Xenon: Can create a swirling, toroidal band of light, often appearing green due to its low resistance.

Gaseous Fountain and Strontium

The "Gaseous Fountain" was a popular Victorian demonstration where a uranium-infused glass (which fluoresces green when excited) was placed in a partial vacuum. As electricity flowed over the glass, it glowed green, while the surrounding excited air emitted a purplish hue, creating a waterfall-like effect of light.

Similarly, a Xenon bulb containing strontium powder can produce vibrant, multi-colored light. When the strontium is agitated and excited by the electrical discharge, it emits photons in a variety of colors, creating a dynamic light show.

Natural Phenomena: Aurora Borealis and Lightning

The same principle of gas excitation explains natural light phenomena. The Aurora Borealis, for instance, is caused by low-pressure oxygen in the upper atmosphere being bombarded by ions from the sun, causing it to emit green light.

Lightning, on the other hand, appears white. This is because the intense electrical discharge superheats the air to extreme temperatures. This superheated air emits light through a process called black-body radiation, similar to how a hot wire glows. The hotter the object, the whiter the light it emits. Thus, the white color of lightning is due to the intense heat it generates in the air, not directly from the electrons.

How Sparks Make Music

The sound produced by a Tesla coil is essentially a rapid series of "bangs." When a spark occurs, it rapidly heats the air, causing it to expand explosively, creating a sound wave. If these "bangs" happen at a specific frequency, our ears perceive them as a musical note. For example, 440 bangs per second produce an A note.

By precisely controlling the frequency and timing of these sparks, Tesla coils can be made to play melodies. This was demonstrated with various musical pieces, including the "Shepherd Tone" (an auditory illusion of continuously rising pitch) and "Flight of the Bumblebee," played by multiple Tesla coils in unison and as a duet.

Wireless Power Transfer

Nicola Tesla envisioned his coils as a means for wireless power transfer. The high electric and magnetic fields generated by Tesla coils can excite gases in nearby bulbs, causing them to glow without direct electrical connection. This demonstrates Tesla's dream of wirelessly lighting rooms.

Recap

The presentation concluded with a recap of the key concepts: - Spark creation: Overcoming the insulating properties of air by either increasing voltage or reducing insulation (vacuum). - Electron origin: The word "electron" comes from the Greek word for amber, referencing early experiments with static electricity generated by rubbing amber. - Voltage amplification: From friction-generated static charge to Faraday's induction and Tesla's resonant coils. - Visibility of sparks: We don't see electrons directly. Instead, we see the light emitted by excited gas molecules (like the purple of nitrogen in air) or the black-body radiation from superheated air (like the white of lightning). - Musical sparks: Tesla coils produce music by generating rapid, controlled "bangs" that our ears interpret as notes.

The evening culminated in a grand finale featuring multiple Tesla coils playing music, accompanied by the dramatic explosions of hydrogen balloons, further illustrating the power and versatility of electrical sparks.