The Weird Physics Behind The Jacob's Ladder, Frankenstein's Favorite Prop

In old movies, when there is electricity involved, there always seem to be two wires fanning out and a slowly-rising arc of electricity between them. This contraption is called Jacob's Ladder, and although it looks uselessly sci-fi, it was actually quite useful in machinery... a hundred years ago.

Jacob's Ladder is a piece of physics equipment that is both simple and showy, and so found itself right at home in Hollywood. It probably found bit parts in a bunch of horror and sci-fi movies, but its starring role was in the 1931 version of Frankenstein. In the famous "It's Alive!" scene, there are any number of electrical devices in the lab, and most of them are pictured as two wires, with an arc of electricity branching up between them until it crackles into the air. There are two in the lab, prominently displayed just the other side of the camera from Frankenstein's body, and there is one on the dais upon which the monster is raised. Their movie arcs of electricity make the scene look active and alive, even when Frankenstein's monster is just a corpse.

What's actually going on? Nothing sinister, but something a little bit dangerous. I certainly wouldn't want to be on a moving platform with a Jacob's Ladder device and a lot of flapping bed sheets. The device consists of two wires held close together. Beneath them is a device that will raise the electrical potential difference between them. Electrons are yearning to jump from one wire to the other, but the air between them acts as an insulator. Eventually, enough voltage will cause electrons to jump away from the air molecules they're associated with. They won't necessarily head to the wire, they'll just be separated from their atoms and molecules a bit. The air has now changed from a gas to a plasma. And the plasma, which is more free with its electrons, conducts electricity. The electrons zoom through the air from one wire to another, and we see an arc of light.

But why does it travel? Since the wires fan out, and the electrons will take the shortest jump they can find, there doesn't seem to be a reason for the electrical arc to move to where the wires are father apart.

The electrons do look for the path of least resistance, but the path of least resistance is always where the plasma is. And the moving electrons heat the plasma around them. This ionized air is less dense than the surrounding air, and moves upwards. The electrons follow it, until at last the jump becomes too far for them, and the connection is lost. Jacob's Ladder is an impressive looking prop, but it needs to be handled with care. The connection can, under the right circumstances, set fire to paper and cloth. Not something to have around an actor's pretty face or sprayed-up hair.

Jacob's Ladder seems pretty useless when it comes to pretty much everything but making movie audiences ooh and ah. It certainly wouldn't have brought anyone back to life — though if they were sleeping, I imagine being set on fire would wake them. Their actual use, back in the early 1900s, was much less glamorous. They ionized the nitrogen in air. Some of that nitrogen would re-combine with oxygen, making nitric oxide. This was used in fertilizer, so it is possible that Jacob's Ladder did 'give life' to something in a round-about way. That wouldn't have impressed Frankenstein, though.

Top Image: Monster Brains

Via UAF, The Physics Book, and UCSC.