The Spark Gap.

A spark gap

A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms, ionizing the gas and drastically reducing its electrical resistance.
An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the 'holding current'. This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas. Usually the action of ionizing the gas is violent and disruptive, often leading to sound (ranging from a snap for a spark plug to thunder for a lightning discharge), light and heat. Spark gaps were used historically in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, and x-ray machines. Their most widespread use today is in spark plugs to ignite the fuel in internal combustion engines, but they are also used in lightning arrestors and other devices to protect electrical equipment from high voltage transients.

Spark visibility

The light emitted by a spark does not come from the current of electrons itself, but from the material medium fluorescing in response to collisions from the electrons. When electrons collide with molecules of air in the gap, they excite their orbital electrons to higher energy levels. When they fall back to their original energy levels, they emit the energy as light. It is impossible for a visible spark to form in a vacuum. Without intervening matter capable of electromagnetic transitions, the spark will be invisible.

Applications

Spark gaps are essential to the functioning of a number of electronic devices.

Ignition devices

A spark plug. The spark gap is at the bottom.

A spark plug uses a spark gap to initiate combustion. The heat of the ionization trail ignites a fuel-air mixture inside an internal combustion engine, or a burner in a furnace, oven, or stove.

Radio transmitters

A spark radiates energy throughout the electromagnetic spectrum. Nowadays, this is usually regarded as illegal radio frequency interference and is suppressed, but in the early days of radio communications (1900-1920), this was the means by which radio signals were transmitted, in the unmodulated spark-gap transmitter. Many radio spark gaps include cooling devices such as the rotary gap and heat sinks, since the spark gap becomes quite hot under continuous use at high power.

Spark gaps as protective devices

Spark gap tube

Spark gaps are frequently used to prevent voltage surges from damaging equipment. Spark gaps are used in high-voltage switches, for example, in power plants and electrical substations. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact and two leaf springs holding the other end as second contact. If the blade is opened, a spark may keep the connection between blade and spring conducting. (The spark ionizes the air, which becomes conductive, allowing an arc to form, which sustains ionization and hence conduction.) Here, a Jacob's ladder (see below) on top of the switch will pull the arc apart and so extinguish it. You might also find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons. These are sometimes called horn gaps. If a spark should ever manage to jump over the insulator and give rise to an arc, it will be extinguished.

Smaller spark gaps are often used to protect sensitive electrical or electronic equipment from high voltage surges. In sophisticated versions of these devices (called gas tube arresters), a small spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground and thereby protecting the equipment. These devices are commonly used for telephone lines as they enter a building; the spark gaps help protect the building and internal telephone circuits from the effects of lightning strikes. Less sophisticated (and much less expensive) spark gaps are made using modified ceramic capacitors; in these devices, the spark gap is simply an air gap sawn between the two lead wires that connect the capacitor to the circuit. A voltage surge causes a spark which jumps from lead wire to lead wire across the gap left by the sawing process. These low-cost devices are often used to prevent damaging arcs between the elements of the electron gun(s) within a cathode ray tube (CRT).

Small spark gaps are very common in telephone switchboards, as the long phone cables are very susceptible to induced surges from lightning strikes. Larger spark gaps are used to protect power lines.

Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications. Neon bulbs are also used for this purpose.

Power-switching devices

Special purpose, high-energy triggerable spark gaps can be used to rapidly switch high voltages and very high currents for certain pulsed power applications, such as pulsed lasers, railguns, fusion, ultrastrong pulsed magnetic field research, and in the triggering of nuclear bombs. Commercially available devices can be divided into two classes: positive pressure and triggered vacuum gaps. Positive pressure triggered gaps have a limited operating voltage range (for instance, from 1/3 to 2/3 of the self breakdown voltage). Triggered vacuum gaps offer a wide operating voltage range (400 V to 90 kV is achievable). Both classes can switch higher energy levels than any thyristor, thyratron, krytron, or sprytron. Triggered gaps are popular for single shot and low repetition rate applications. One such switch is known as a trigatron. The Ignitron and Crossatron could be considered a triggered gaps. The latter is unique in that it can be turned back off by the control electrode after conduction begins. The xenon flash tube is another common triggered gap. Various schemes have also been devised to trigger open air gaps on command. A set of spark gaps are a key element of a Marx generator, used to generate high-voltage impulses; the spark gaps allow a chain of capacitors to be slowly charged in parallel and then rapidly discharged in series.

Visual entertainment

A time exposure of a Jacob's Ladder

A Jacob's ladder (more formally, a high voltage traveling arc) is a device for producing a continuous train of large sparks which rise upwards. The spark gap is formed by two wires, approximately vertical but gradually diverging away from each other towards the top in a narrow "V" shape. It was named for the "ladder to heaven" described in the Bible.

When high voltage is applied to the gap, a spark forms across the bottom of the wires where they are nearest each other, rapidly changing to an electric arc. Air breaks down at about 30 kV/cm, depending on humidity, temperature, etc. Apart from the anode and cathode voltage drops, the arc behaves almost as a short circuit, drawing as much current as the electrical power supply can deliver, and the heavy load dramatically reduces the voltage across the gap.

The heated, ionized air rises, carrying the current path with it. As the trail of ionization gets longer, it becomes more and more unstable, finally breaking. The voltage across the electrodes then rises and the spark re-forms at the bottom of the device.

This cycle leads to an exotic-looking display of electric white, yellow, blue or purple arcs which is often seen in films about mad scientists. The device was a staple in schools and science fairs of the 1950s and 1960s, typically constructed out of a Model T spark coil, or any other source of high voltage in the 10,000–30,000 volt range, like a neon sign transformer (5–15 kV) or a television picture tube circuit (flyback transformer) (10–28 kV), and two coat hangers or rods built into a "V" shape. For larger ladders, microwave oven transformers connected in series or utility pole transformers (pole pigs) run in reverse (step-up) are used.

Traveling-arc devices are dangerous. The sparks can burn through thin paper and plastic and start fires, and contact with the exposed high-voltage conductors can be lethal.

Health hazards

Exposure to an arc-producing device can pose health hazards. In a closed space such as a classroom or home, the continuous arc formation of an open-air Jacob's Ladder will ionize oxygen and nitrogen, which then re-form into reactive molecules such as ozone and nitric oxide. These free radicals can be damaging to the mucous membranes of people near the spark gap. Plants are also susceptible to ozone poisoning.

These hazards are not present when the arc is formed outdoors since the heated ionized gases will rise up into the air and dissipate into the atmosphere. Spark gaps which only intermittently produce short spark bursts are also minimally hazardous because the volume of ions generated is very small.

Arcs can also produce a broad spectrum of wavelengths spanning the visible light and the invisible ultraviolet and infrared spectrum. Very intense arcs generated by means such as arc welding can produce significant amounts of ultraviolet which is damaging to the retina of the observer. These arcs should only be observed through special dark filters which reduce the arc intensity and shield the observer's eyes from the ultraviolet rays.