Jacob's Ladders

Sci-fi action right at home

 5 Jun 2004, 24 Oct 2003 

Jacob's Ladders

What is a Jacob's Ladder?

A Jacob's Ladder is the type of high voltage "climbing arc" "V" shaped display seen in many old Sci-Fi movies. Jacob's Ladder come in all shapes, styles, and sizes. It involves basically the use of relatively high voltages and currents.

How does it work?

The simple explanation is that an arc starts at the bottom, and due to the fact that hot air rises, the arc tends to move up the diverging rods until they are too far apart for the voltage/current provided by the power source.

Now lets analyse it. As from Sam's Powerlab's page, the term voltage refers to potential, or like water pressure. Current basically refers to electrical flow, or like the volume of water flowing through a pipe. A dielectric is something that resists the flow of electricity until a high enough voltage is created, sort of like fish clogging a pipe until the water pressure builds enough to blast the fish and rush through. Okay, Air is a dielectric, and when two points build up enough voltage between them, a spark jumps between them, like lightning. Air is a fairly tough dielectric, and 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. In a Jacob's ladder, both current and voltages are usually quite high, so at anything, do not touch the ladder when it is in operation!

However, it is not just the fact that 'hot air rises' that contributes to the effect. 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. I have tried for both AC and DC currents and they both work.

Build a Jacob's Ladder

Basic Components - Power Supply

Jacob's ladders are relatively easy things to construct. There are only two major parts to a basic Jacob's Ladder: a high voltage power source and a pair of rods arranged in a narrow V configuration on an insulated and fireproof support. You will need at least 6kVAC at 20mA or higher. However, the exact values are not at all critical. Lower voltages work, but not reliably. A neon sign transformer is the usual source for this power though an oil burner ignition transformer will too, or you could build an inverter type power supply.

  • Neon Sign Transformers (NST) can be obtained used from sign shops or metal recycling companies. The cost will be anywhere from free to $150 or more depending on size and condition and whether the seller has a use or other buyers for this sort of equipment. Typical ratings: 7,500 to 15,000VAC current limited to 20 to 30 mA. Larger ones are available - up to 60 mA or even 120 mA. There are smaller transformers from 2,000V to 6000V but they do not produce a very impressive display. When getting a neon sign transformer, those heavy iron-cored transformers perform better and are more study than electronic transformers (which can't quite handle much abuse).  

  • Oil burner ignition transformers (OBIT) can be removed from discarded oil burners, and can be obtained cheaply from junkyards etc. However, you will likely have to disassemble the disgustingly icky burner assembly and properly dispose of the unwanted parts as part of the deal. Typical ratings: 5,000 to 10,000VAC, current limited to 10 to 25 mA.  

  • Other lower powered devices like automobile ignition coils and television flybacks will work too. A small jacob's ladder is just as fun. Ignition coils can provide various voltages and currents depending on the power fed to it. Flybacks generally provide quite high voltages but at low currents. They provide an interesting display too. Laser power supplies can be used as well.

There are some other power supplies, which I do not recommend you use.

  • Microwave Oven Transformers (MOT) produce only about 1,500 to 2,500 VAC which is too low. Several in series would be required but this is an extremely dangerous and unwieldy arrangement. They are not current limited like NSTs or OBITs, and can put out up to or more than 2 Amps on the high voltage side, and will blow your circuit breakers. They are LETHAL and WILL kill instantly due to the extremely high currents. Save yourself and your house circuit breaker from popping like pop-corn and do not use MOTs.

  • Utility Pole and substation distribution transformers. Aside from requiring a forklift of or a 10 ton crane to move it, it will probably blow your circuit breakers unless some heavy-duty current limiting is installed. These are rated from 10kW to 25kW and are even more dangerous than your electric chair... though some experienced people still build using these for very impressive displays.


Take a pair of thin metal rods - the steel wire from old metal coat hangers works quite well. You can also use some thick steel or copper wire. Straighten them out and mount them on an insulated non-flammable support with a gap of about 1/2 inch at the bottom and 1 to 3 inches at the top forming a narrow tall 'V'. This depends on your power supply and you'll need some experimenting to get the optimum positions. Mounting locations should not be in the path of the rising arc. Connect the high tension output of the transformer to the two rods using high voltage insulated wire unless the routing is such that there is no chance of arcing where you don't want it. Some adjustment of the spacing at the bottom (to get the arc started) and at the top (to determine when the arc is extinguished and how fast it rises) may be required (but do so only with the power tuned off!). Depending on the voltage and power rating of your high voltage source, these dimensions may vary considerably. Spirals and other more creative configurations are also possible, but are more difficult to construct.

A Jacob's Ladder works on the principle that the ionized air in the arc is a lower resistance than the air around it and heated air rises. The arc strikes at the point of lowest breakdown voltage - the small gap at the bottom. The heated plasma rises and even when it is an inch or more in width is an easier path for the current to follow. Eventually, the gap becomes too wide, the arc extinguishes and is reestablished at the bottom. For best results, shield the whole thing from drafts but don't use anything that can catch fire! It's possible to use a large diameter clear acrylic or lexan tube to shield the whole thing.

Basically it should look like this. That's about as simple as it can get. You can make your own base etc, like what I did.

Gabriel's Electrode to assist arc striking

The gap between the electrodes at the bottom of a Jacob's ladder can be extremely critical. Too wide and the arc won't strike, and too narrow and it will just stay at the bottom. If you cannot acquire a high enough voltage transformer, the distance becomes even more critical. This electrode would assist arc striking.

It's simply a third electrode placed between the strike gap at the bottom of the "V". It is connected to either one of the main electrodes via two 1M ohm high voltage resistors. When an arc should occur, the following happens...

1. The voltage on the middle electrode floats to the potential of the electrode it's connected to via the resistors.
2. It's easy for an arc to jump the short distance from the other electrode to the middle one.
3. When an arc has struck and current is flowing, the voltage on the middle electrode flies up due to the high resistance value.

The combination of high voltage at the middle electrode and the ionized path makes the arc strike all the way across. It's simple, but works perfectly.


My Ladders

Here are some of the Jacob's ladders I have built, ranging from small flyback driven ones to MOT driven ones.

Flyback Driven

This is my first Jacob's ladder. It's extremely small and powered by a small Television flyback transformer (using a 50W lighting transformer... Made simply with a small plastic box, and two bent wires.

Visit my High voltage flyback page for more information.

Ignition Coil Driven

This is my second ladder, pieced together in a hurry. (click for larger image)

Higher voltage, but relatively lower current than NST. Check it out the ignition coil driver at my ignition coil page.

Neon Sign Transformer Driven

The mother of my Jacob's Ladders! Standing about 70cm tall, constructed with bolts, nuts, and acrylic... the large Jacob's ladder.

It's powered by a 7.5kV 30mA Neon Sign Transformer with a Gabriel's Electrode.

Above is an animated sequence showing how it going up the ladder. The grey block is the NST. Took about 1 hr to drill and construct the base. The base is a bit small though, and it's a bit wobbly, but this ladder is totally spectacular and the sound is just amazing. It's a sure show-stopper at parties.

Below are more pictures of the construction and the ladder in action.

In the first picture you can see the base construction and the electrode in action. Although simple, it's extremely effective and works like a charm. This ladder is obviously much more high powered than my previous ladders, and leaves the the ladder blistering hot. Always remember to let the rails cool before adjusting it. Make sure it can't fall down during operation! Adding different salts on the rails changes the arc colour.


Video Clips

This would not be complete without a video of the ladder in action. It's much more amazing in reality.
Download the video: ladderbig.wmv (571kb. Requires Windows Media Player 7 or higher to view)

Powerful MOT Driven Ladder - The ...father(?) of my Jacob's Ladders!

Here is my MOT driven jacob's ladder setup. (click picture to enlarge) Although it's not very tall, it has a more distinct "V" shape. The top is around 9cm wide.

It's powered by a 5.5kg Microwave oven transformer from a 1.1kW oven. When shorted (in jacob's ladder use), it should draw about 2000W from the mains. I don't have a suitable measuring device so I cannot measure the output. Output voltage would be around 2kV but at extremely high current! (1A) If you can remember my explanation above, 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. 2kV is way too little for a ladder to start consistently by itself, therefore, I have made a simple trigger rod, basically, a piece of wire taped to a wooden rod.

Assuming 2000W draw, that works out to be 2kV at 1 Ampere (1000mA)! which is Extremely lethal. This high current allows the arc to be drawn very wide. The base is made of clear acrylic (surplus piece) and the electrodes are made of thick steel wire. Stainless steel would be more ideal but it is hard to obtain. At these power levels, electrode erosion becomes a real problem. (This ladder is about 2000W, which is significantly higher power than my neon sign transformer jacob's ladder, which is only 225W. That's almost ten times more power!) However, the display is spectacular...

And more pictures of the ladder...


The base construction if the ladder in the first picture. Note the terrible electrode erosion in the second picture.

And the ladder in action.

This is a frame caught from a movie clip. The arc is VERY bright and HOT. The ladder gets Extremely hot even after a short run of a few seconds, and the bottom of the electrodes can get RED HOT after long runs. Eye protection is a must. Remember, this is an EXTREMELY dangerous setup due to the high currents involved. (even though voltage is fairly low, but then again, 2000V can jump an air gap). Keep it well away from you when operating ladder like these and always turn off the power and allow the ladder to cool for several minutes before tuning or adjusting the electrode spacing.

Video Clips!

This would not be complete without a video of the ladder in action. It's totally awesome in reality!.
Download the video: motladder.wmv (798kb. Requires Windows Media Player 7 or higher to view)

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(c) Gao Guangyan 2011
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