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A 14^th century carrack quietly sails through the currents of the Atlantic Ocean in the middle of the night. Its navigation relies on the stars shining above, its power on the wind blowing behind. It’s a far cry from the technologically advanced vessels sailing today’s seas.

It was here, long before civilisation began using and generating electricity, that the ghostly, blue-white glow of electricity acting upon air molecules was often seen as it hovered around ships’ masts.

This phenomenon is known as St. Elmo’s Fire, after Saint Erasmus of Formia – the patron saint of sailors – and occurs following thunderstorms when the electric field still present around an object (such as a lightning rod or a ship’s mast) causes air molecules to break up and become charged, creating what’s known as a plasma.

St. Elmo’s fire on a cockpit window

For sailors in an era long before satellite guidance it was a good omen. What they didn’t realise, however, was it was one of the rare instances when electricity acts in a way that changes it from an unseen force to something we can see, hear and even smell.

The science behind seeing electricity

Normally, we can’t see electricity. It’s like gravity – an invisible force we only recognise when it acts upon other objects. In the instance of electricity, the most common way it affects objects is by charging electrons, and because these are so small, so plentiful and move so quickly once charged, they are all but invisible to the naked human eye.

However, there are instances when conditions enable an electric current to conduct through the air, which can create sound and generate a visible plasma.

“You can see electricity in certain instances because it’s ionising the air,” says Drax Lead Engineer Gary Preece. In the process of ionising, the molecules that make up air become electrically charged, which can create a plasma.

“The electric current is able to bridge the air gap through the ionised air and to earth,” explains Preece. “You need to have that path to earth for it to create a spark.”

It’s a similar process to how a spark plug works or how lightning becomes visible. While there is still scientific debate around how clouds become electrically charged, the flashes seen on the ground are caused by discharges between clouds, or from clouds to the earth, creating a very hot and bright plasma.

The atmospheric conditions of our earth being largely oxygen and nitrogen give lightning a whitish-blue colour, like St. Elmo’s Fire. Altering these conditions so electricity passes through a gas such as neon changes the colour to a red-orangey shade, which is the principle on which neon lights and signs are built. To achieve different colours, different gases such as mercury and helium are used to fill the tube.

Long before we learned how to manipulate electricity to create different coloured signs we were battling with how to create visible, useful electricity. And it began with the use of arcs.

The architecture of electric arcs

Electric arcs occur when an electric current bridges an air gap. While air is an insulator, electricity’s constant attempts to conduct to earth sometimes enable it to find paths through it, ionising the air molecules and creating a visible plasma bridge along the way. The higher the voltage, the greater the distance it can arc between electrodes.

This property of electricity presents dangers such as arc ‘flashes’, which can occur during electrical faults or short circuit conditions and expel huge amounts of energy, sometimes creating temperatures as high as 35,000 degrees Fahrenheit – hotter than the sun’s surface.

When controlled, electrical arcs can be very useful. These bright glowing bridges were used in the first practical electric lights after Humphry Davy began showcasing the technology in the early 19^th century.

But the need to replace carbon electrodes frequently, their buzzing sound and the resultant carbon monoxide emissions meant the technology was soon replaced with the incandescent bulb.

Today arcing is used in welding and in more sophisticated plasma cutting, which focuses a concentrated jet of hot plasma and can make highly precise cuts, while arc furnaces are used in industrial conditions such as steel making.

In fact, some thought has even gone into how we could use an incredibly powerful beam of plasma to create a working lightsabre. And although compelling, the theory of creating this super advanced Star Wars technology is far from being a practical possibility.

In the 14^th century seeing electricity was a rare and positive omen. Today, seeing electricity has become far more common, yet when it does happen – through plasma spheres, neon lighting or naturally occurring lightning – the effect is the same: human wonder at seeing an awe-inspiring and seldom-seen force.

On cold, misty mornings, the powerlines, pylons and transformers that make up Great Britain’s electricity system sometimes sound a little different. Stand in a field under a powerline and, in the right conditions, the usual sounds of the countryside might be interrupted by the crackling of electricity.

This buzzing crackle, which can be referred to as a corona discharge, occurs when there’s a change from the normal conditions of a power line’s insulators enabling the electric current to partially conduct along it or through the surrounding air to earth. This can come as a result of weather conditions or deterioration of the insulators.

It’s just one of the instances where electricity changes from an unseen force powering our lights and devices to something we can hear, see and even smell.

Why can we hear electricity?

The source of electricity’s sound is also determined by one of its inherent properties: frequency. Frequency is the measurement of multiple occurrences from events, such as sound waves from vibrations, over a period of time. Any equipment that has a frequency causing mechanical parts to undergo repeated change can be audible.

For example, if we hit a cymbal with a drumstick we can hear a crash because of the frequency of the vibrations the mechanical part (in this case the cymbal) makes. We hear a guitar because its strings are plucked and pulled to create vibration at different frequencies. And we can hear an audible hum in a transformer because electric currents affect its internal structure and cause vibrations.

The buzzy tone this creates can be referred to as ‘mains hum’ and is ever present, although not always perceptible to the human ear. It becomes audible, however, when electricity (specifically alternating current – AC) is applied to a transformer.

Transformers are made of lengths of copper or aluminium, which are wound around a steel laminated core. When AC is applied it magnetises the core and causes steel laminations in the transformer to expand and contract in a process known as magnetostriction. Although only small physical changes, these movements are enough to cause vibration, which in turn creates an audible hum.

The crackling overhead

With more than 7,200 km of overhead powerlines humming away constantly around the country, corona discharges are inevitable and common.

This happens when part of the insulators on a high-voltage line begins to deteriorate, is exposed to weather conditions, is damaged or contaminated, allowing electricity to partially flow along it. The surrounding air becomes electrically charged through a process known as ionisation and causes molecules to become charged and collide.

It’s these collisions in the air that make the corona audible. It can also be visible and gives off a distinct smell of ozone, the gas produced when air is ionised.

Although not dangerous to someone on the ground below, if the insulator on the powerline is left to deteriorate further it can fail completely, leading to earth faults that trip power systems.

Making use of electric hums

The sounds electricity creates may seem like a nuisance but they can also have important uses. One of the most innovative applications is in forensic analysis.

A technique called Electric Network Frequency (ENF) enables forensic scientists to validate audio recording by observing the frequency of the mains hum picked up by audio recordings, which is not audible to humans.

By comparing the frequency of the hum in a recording to a database of the country’s frequencies at given times, it’s possible to verify when and in which country the recording took place and detect any editing.

It highlights not only an innovative use of electricity, but just how pervasive and consistent a presence it is. So, while the ebb and flow of electricity through our lives often goes on without thought, it is always there, humming away while it powers modern life in Great Britain.