What is a Jacob’s Ladder?
A Jacob’s Ladder is the type of high voltage ‘climbing arc’ display seen in many old (and usually bad) Sci-Fi movies. Jacob’s Ladder come in all shapes, styles, and sizes.
How Does a Jacob’s Ladder Work?
The high voltages involved allows the electric current to leap through the air to a nearby contact point. The air acts as an dielectric, resisting the flow of electricity. When a high enough voltage is built up between two points, a spark jumps through the air like lightning. Through air, it takes about 1 thousand volts to jump 1.1mm. The voltage determines the distance the arc can first strike the other electrode. The current on the other hand determines the distance the arc can be drawn after it has connected the two electrodes.
The simple explanation is that an arc starts at the bottom and due to the fact that hot air rises, tends to move up the diverging rods until they are too far apart for the voltage provided by the power source.
While it is true that warm air pushes the arc up the ladder, there is also the typical ‘high leakage’ or reactance curve of the transformer contributing to the effect. The transformer will happily arc across the bottom as long as Paschen’s Law will allow. Once this arc is struck the current in the arc will actually increase to the transformer’s preset limit. The heat is also creating higher resistance.
Normally the transformer would try choke the voltage down as current increased. But just above the arc exists a path that the transformer can easily maintain and which in fact will lower its current.
At the top we are not only at the upper limit of the transformer but it is also where the current is very low and so all the fun breaks apart only to reignite down below.
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1. High Voltage Transformer
2. Metal Rods
3. Insulating Base
4. Connecting Wires
There are only two major parts to a basic Jacob’s Ladder: a high voltage power source and a pair of metal rods arranged in a narrow V configuration on an insulated and fireproof support.
A few types of transformer can be used. Generally, amateurs look for a few types of transformers. These range from luminous tube transformers to microwave oven transformers, but I’m only going to comment on the type I use: Neon Sign Transformers.
Neon Sign Transformers (NSTs). They can be obtained from neon sign shops. The cost can be from $30 to over a $100, depending on the condition and ratings. They are generally ranged from 6kV to 15kV, with about 30mA. There are 2 kinds of Neon Sign Transformers, one of them is Iron cored and run at 50Hz, the other are the new smaller switchmode ones which run at 20kHz and are a lot lighter. The heavy iron-cored ones usually perform better.
The metal rods could be anything. A length of thick copper wire would do, so would thin copper pipes, and even the metal frame from old metal hangers.
The general design is as follows:
Just like that. As simple as it can get.
For the first attempt, I used the metal frame from old metal hangers. I removed the outer layer with a pair of scissors and some hard work. The metal was terribly rusty, so I spent quite some time sanding it.
I cut the metal frame into two lengths, and bent them to shape. They were held down to a plastic support with random weights I found around my house.
Nothing is tuned and adjusted to produce the best looking Jacob’s Ladder, but it’s just a proof of concept.
And power’s on!
I decided to have a more permanent setup, something that I can have on display when there’s ever a need to. It’s gotta be much aesthetic and steadier than the first attempt, so I came up with a proper design.
I went out to purchase a small piece of acrylic, and a 2m long flat thin metal bar. The bar’s made of brass, and I had it cut into a few pieces. They would serve as the metal rods. Also, they’re rather expensive actually, and I felt guilty after purchasing them.
It took some difficulty to bend them as they’re quite tough, but it worked out eventually.
I drilled holes in the acrylic and the metal bars, to attach bolts and nuts through them. These bolts and nuts will secure the metal bar firmly to the acrylic. Drilling large holes through metal is such a pain…
I did a few runs, adjusting the positions of the metal bars to produce the optimal spark. After 2 or 3 runs, I finally decided on this:
And the results!