How Sparks Are Created, Seen, Heard, and Turned into 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. The presentation, led by Michael and Professor Ricketts, focused entirely on live demonstrations, eschewing slides for a more engaging experience.

How Sparks Are Made

The fundamental question addressed was how to create a spark. Air, though seemingly empty, is a good insulator. To force electricity through it, 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, overcoming the resistance of gas molecules. When the voltage is sufficient, electrons cascade in an "avalanche," creating a low-conductance path of ionized air, through which the spark flows.

Static Electricity and Lightning

The simplest 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 produce 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, a giant spark—a lightning bolt—occurs, involving millions of volts.

Generating 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 exploiting electromagnetic induction. Michael Faraday discovered that a change in a magnetic field induces a high voltage.

1. Magnetic Field Creation: When current flows through a wire, it creates a magnetic field around it.
2. Interruption of Current: If the current is suddenly interrupted (e.g., by breaking a connection), the magnetic field collapses.
3. Induced Voltage: The collapsing magnetic field induces a very high voltage, which can be tens of thousands of volts, causing a spark to jump across the gap where the connection was broken. This is why sparks are often seen when disconnecting electrical wires, not just connecting them.

The Induction Ring and Transformer

Faraday's induction ring, an early transformer, demonstrated how to amplify voltage. It consists of two coils of wire. By passing current through one coil (the primary), a magnetic field is created, which in turn induces a current in the second coil (the secondary). If the secondary coil has more turns than the primary, the induced voltage will be higher. Faraday meticulously insulated his wires with silk by hand for his original 75-foot coils.

Modern transformers, like those found in black tubes, use this principle. They have a primary coil with fewer turns and a secondary coil with many more turns, often wound around an iron core to concentrate the magnetic field. 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.

Tesla Coils and Resonance

Nikola Tesla advanced this concept with the Tesla coil, which utilizes resonance to generate extremely high voltages.

1. Components: A Tesla coil has a primary coil (fewer turns) and a secondary coil (many turns of fine wire), often with a large metallic sphere or toroid at the top.
2. Resonance: Unlike a simple transformer, a Tesla coil is designed to resonate. The primary and secondary circuits are tuned to the same resonant frequency.
3. Analogy to a Pendulum: Just as a small push can make a pendulum swing very high if timed correctly, a small electrical "perturbation" from the primary coil, amplified by the transformer action, can build up enormous voltages in the resonant secondary coil.
4. Electrical Resonance: In an electrical circuit, potential energy is stored as voltage (like a capacitor), and kinetic energy is stored as current (like an inductor). The Tesla coil's design allows these energies to oscillate back and forth, building up immense voltage. The top sphere acts as a capacitor, storing charge relative to the rest of the world.

A demonstration with a miniature Tesla coil showed it could produce sparks spanning 10-15 cm, and a larger one, named "Jerry," could generate sparks of 20-30 cm.

What We See When We See a Spark

The presentation then explored what is actually visible when a spark occurs.

The Invisible Electron

Electrons themselves are invisible. A demonstration using a vacuum tube (Maltese cross tube) with almost all air removed showed that while electrons flow and create a shadow on a phosphorescent screen, the flow itself is not directly visible. Electrons are thousands of times smaller than the wavelengths of visible light.

The Role of Gas and Excitation

If electrons are invisible, what causes the light we see in a spark?

1. Partial Vacuum: In a partial vacuum, where some air molecules remain, the flowing electrons collide with these gas molecules.
2. Excitation and Emission: These collisions excite the electrons within the gas atoms, causing them to jump to higher energy levels (shells). When these excited electrons fall back to their original energy levels, they release energy in the form of photons, which we perceive as light.
3. Color Variation: The color of the emitted light depends on the type of gas and the energy difference between the electron shells.
   • Air (Nitrogen): In a partial vacuum, air (mostly nitrogen) produces a light purple color.
   • Neon: Emits a yellow/orange glow.
   • Argon: Produces a different, amorphous glow.
   • Xenon: Creates a spinning, toroidal band of color, often appearing green due to its low resistance.
4. Gaseio's Fountain: A historical demonstration using uranium-infused glass in a vacuum chamber showed how electrons could excite a solid material, causing it to fluoresce green as the charge flowed over its surface like a waterfall.
5. Strontium Powder: A Xenon bulb containing strontium powder demonstrated how shaking the bulb could cause the strontium salts to be excited by the electricity, emitting photons in a variety of vibrant colors.
6. Aurora Borealis: This natural phenomenon is an example of low-pressure oxygen in the upper atmosphere being excited by ions, emitting green light.

White Sparks and Black Body Radiation

While many sparks show distinct colors due to gas excitation, very intense sparks, like those from a powerful spark gap or lightning, appear white. This is due to "black body radiation."

1. Heating the Air: The intense energy of a powerful spark superheats the air in its path.
2. Incandescence: This extreme heat causes the air to glow incandescently, similar to a heated wire. The hotter the material, the whiter the light it emits.
3. Lightning: Lightning is white because it superheats the air, emitting bright white black body radiation.

Therefore, the light we see in a spark is not the electrons themselves, but the response of the surrounding gas or material to the flowing electrons.

How Sparks Make Sound (and Music)

Every time a Tesla coil operates, it produces a distinct sound. This sound is directly related to the rapid heating and expansion of air caused by the spark.

1. Rapid Heating: The spark superheats the air in its path almost instantaneously.
2. Expansion and Shockwave: This rapid heating causes the air to expand explosively, creating a shockwave that we hear as a "bang."
3. Music from Bangs: If these "bangs" are produced at a specific frequency, they can create musical notes. For example, 440 bangs per second are perceived as an A note. Tesla coils can be modulated to produce a series of rapid bangs at varying frequencies, thereby playing music.

The presentation concluded with a musical performance by two Tesla coils, "Tom" and "Jerry," playing a duet, demonstrating wireless power transfer by lighting up gas-filled globes placed near the coils.

Recap

The evening's key takeaways were:

• Spark Creation: Sparks are created by overcoming the insulating properties of air, either by removing the insulator (vacuum) or by significantly increasing the voltage.
• Electron Discovery: The word "electron" comes from the Greek word for amber, referencing early observations of static electricity generated by rubbing amber.
• Voltage Amplification: From simple friction to Faraday's induction and Tesla's resonant coils, methods have evolved to generate increasingly high voltages.
• What We See: We do not see electrons directly. The light from sparks comes from the excitation and relaxation of gas atoms (or other materials) by the flowing electrons, or from the incandescent heating of the air (black body radiation) in very intense sparks.
• Sound and Music: The sound of a spark is caused by the rapid heating and expansion of air. By controlling the frequency of these "bangs," Tesla coils can produce music.

The finale involved the two Tesla coils playing music while simultaneously igniting hydrogen-filled balloons, combining the visual spectacle of sparks with explosive sound and musical performance.