Sky voltage

Sky voltage refers to an electrostatic voltage gradient that is present in the free air of the atmosphere, and which can have a different voltage potential relative to the surface of the planet.

Tall insulated metallic objects can absorb these voltage gradients out of the atmosphere, and conduct the voltage to a terrestrial collection and measurement station.

Small lightweight electrostatic motors and toys can be driven from this collected sky voltage when one side is connected to earth ground, and the other is connected to the sky collector.

Sky voltage and the electrical neutral wire

Sky voltage was a problem for early electrical power and communications grids which used suspended insulated bare wire to transport current. These suspended wires also acted as sky voltage collectors, and could result in dangerous sparking from electrical devices to nearby earth grounded metal, or sparking to the people touching the electrical devices. Due to having wires spanning tens and hundreds of miles, the wire has the capacitive potential to store large charge quantities across the system that could prove lethal to energy customers. Lightning striking near an electrical power device also had the potential to leak dangerous high voltages into the system that could spread throughout the system in moments.

In order to safely dissipate this collected sky voltage, one current-carrying wire of the electrical grid is connected to an grounding rod at regular intervals throughout the power system. This serves to discharge and bleed off any large voltage charges being collected from the suspended wires. This wire is called the "neutral", since its current flow is zero, relative to the earth ground.

Modern three-phase transmission lines also include a fourth suspended grounding wire which carries no load, but helps to absorb the sky voltage and prevent electrostatic buildup in the other polyphase current-transporting wires.

See also

*Static electricity
*Electrostatic motor
*Leyden jar