Jacob's Ladder

A Jacob's ladder is a high-voltage display device in which a visible electric arc travels upward between two stiff wire electrodes that diverge in a narrow V shape. Connected to a high-voltage source, the gap at the bottom is narrow enough that the air breaks down and an arc forms across it. The name comes from the biblical ladder reaching toward the sky, reflecting the way the arc appears to climb.

Once the arc strikes at the closest point, it ionises and heats the surrounding air. The hot, ionised air is less dense and rises, carrying the conductive channel with it. As the arc moves up the widening electrodes, the gap grows and the arc stretches, until it can no longer be sustained and extinguishes at the top. A fresh arc then forms again at the bottom of the gap, and the cycle repeats, giving the characteristic rising and snapping motion. The effect is largely a curiosity and a teaching demonstration of electrical breakdown rather than a practical device.

The display relies on a source capable of producing tens of thousands of volts. Traditional sources include neon-sign transformers, oil-burner ignition transformers and other high-voltage transformers, while induction coils and similar arrangements can also drive the arc. Because the voltages and currents involved are dangerous and can produce ozone and a fire risk, Jacob's ladders are operated with care and suitable insulation. They remain popular in science demonstrations and have long appeared as a visual shorthand for electrical laboratories in films.

Frequently asked questions

Why does the arc climb upward?

The arc heats and ionises the air around it. This hot air rises and carries the conductive arc channel up the diverging electrodes until the gap becomes too wide to sustain it.

What powers a Jacob's ladder?

A high-voltage source delivering tens of thousands of volts, such as a neon-sign or oil-burner ignition transformer, or an induction coil.

Is a Jacob's ladder dangerous?

Yes. It uses lethal high voltages, generates heat and ozone, and poses a fire and shock hazard, so it must be built and operated with proper insulation and caution.