How to Build a Five-foot-tall Jacob's Ladder





Introduction: How to Build a Five-foot-tall Jacob's Ladder

About: Eric J. Wilhelm is the founder of Instructables. He has a Ph.D. from MIT in Mechanical Engineering. Eric believes in making technology accessible through understanding, and strives to inspire others to lear...
This classic climbing arc completes any mad scientist's dungeon. Don't touch the electrodes: they're at 12 kV!

Step 1: Obtain Neon Sign Transformer

Get a neon sign transformer rated for 9 - 12 kV and 30 mA. Make sure it's an old style, heavy coil transformer and not solid-state. The solid-state transformers won't start the arc.

I got a Transco 12 kV 30 mA transformer on Ebay for $35. It didn't have a wall plug, so I wired one on.

Step 2: Build a Base

Build a base from the something nonconductive and nonflammable. I chose wood, which doesn?t quite meet the nonflammable criteria. Drill two holes approximately 0.5 inches apart and shove two dowels in the holes. Use thick wire to make electrical connections between the output of the transformer and the dowels. Attach the base to the transformer to make the whole thing stable.

Step 3: Mount Copper Pipes

Mount two copper pipes on the dowels. Make the fit snug by putting the wire between the dowel and the pipe.

Connect the transformer output to the stiff wire.

Step 4: Set the Spacing

Set the spacing at the top of the copper pipes. I used a laser cut piece of acrylic. Something like drywall could also work. Eventually, the spark hung out under the acrylic long enough to ignite it. In later versions I put a gap between the two holes to allow the spark to pass through.

You have to experiment with the spacing; start around 2 inches and go as wide as you can.

Step 5: Build a Case

To make sure no one grabs the pipes, build a case around them.

I cut pieces of acrylic on a table saw and attached them to the base and a small frame at the top. There is a piece of reflective mylar on the back acrylic.

Step 6: Power Up and Take Pictures

Power up and check the spacing. Make sure to unplug before you adjust any gaps!

Use a long exposure to take pictures.



    • Casting Contest

      Casting Contest
    • Oil Contest

      Oil Contest
    • Make it Move Contest

      Make it Move Contest

    We have a be nice policy.
    Please be positive and constructive.




    About 25,000 pulses a second.

    Soon I will make the Jacob ladder and use its RFI to attempt to kill bacteria. I am very creative. It may take a few weeks to get here. You have to be extremely careful and prudent with high voltage especially anything past 10 ma. Mine is a 8 kV, 20 mA at 25 khzs.

    This mean in one second it fires about 1.7 pulses every second so in 1 minute the transformer and the jacob ladder will relase approx 102 pulses.


    I was thinking about making one of these and having it horizontal instead of vertical. Would this work, or does it need to be vertical in order for the spark to travel along the length of the copper pipes?

    9 replies

    It needs to be vertical. The spark heats and ionizes the air, which reduces its resistance. This heated air tends to rise drawing the spark up. You can blow the spark around, but doing so in a consistent manner would be a challenge.

    Maybe a setup using a computer fan, something quite gentle, it would have to reacht the length of the arc rods though, you could use the power supplies case fan as a source of vaguely warm moving air, if it was a little warmer than room temp it would lower the heat gradient needed to make the arcs move, I think, cold air might have a more pronounced effect for a very powerful and slow moving arc...


    Hmm, that might work. If I give it a try I'll be sure to let you know.

    It should work ok if you can get the right airflow...

    This would be very interesting if done inside a vacuum

    In a vacuum, there's no air to ionize; the spark probably would not start at all, and would never move from the point of shortest path if it did. And the electrodes would have to be MUCH closer together in order to get a spark to jump the gap.

    I know your reponse is old but I feel compelled to chip in: You're quite wrong, I promise you. In a vacuum the spark can travel much farther. I have, in fact, tested the theory. I had a 10,000v transformer capable of approximately 1.5" spark. It was managing around 6" in a -25in.Hg vacuum.

    Having air entering top and bottom seems to help the spark rise, making a kind of chimney i suppose.


    Okay, thank you very much.

    saw a news story today where a kid died building one of these, but I don't see any safety comments anywhere. What, exactly, do you do to keep this device from killing the kid you're building it for?

    1 reply

    Step 5: Build a case

    To make sure no one grabs the pipes, build a case around them.

    I cut pieces of acrylic on a table saw and attached them to the base and a small frame at the top. There is a piece of reflective mylar on the back acrylic.

    I bought a Franceformer brand transformer off of ebay for around $40 that I believe was 12kv 30ma (Got my build info from‎). It had the ceramic insulators on each end and the three threaded stud power connections on one side. I bought a three prong plug and a power strip (for the on off switch) along with 12 feet of solid copper wire (I forget what gauge, but it was about the thickness of a pencil) and some connections for connecting the copper/coat hanger electrodes to the transformer. The associate in the electrical section of Home Depot advised me on how the plug would wire up to the transformer. I wired it up and used two coat hangers to make electrodes just to make sure it would work, then I painstakingly straightened the copper wire and made two copper electrodes that were about 6ft tall. They were so tall, that when I turned on the ladder, they were attracted to each other and it messed with the climbing arc. I solved that problem by putting a notched piece of plastic at the very top of the electrodes to keep them steady. It was awesome! Even with the much smaller coat hanger electrodes. I have since lost the transformer and copper electrodes, but I will make another one eventually and I would like to build a nice box for the transformer and plexiglass enclosure for the electrodes so the kiddos can't touch the pretty arc. I like the idea of a steam-punk inspired ladder. I'll be lucky just to get a regular box and electrode enclosure built. I like that you used copper pipe for the electrodes. That takes the bending aspect out of the equation. I'll probably go with solid copper wire again though because it reminds me of the old school ladders from the horror/sci-fi movies.

    1 reply

    Megavolt do you remember how you wired up your 110V supply. It sounds like I have a similar transformer, and I was planning to use a three wire supply, but wasn't sure what to do with the ground. I know better than to connect with the neutral, which only leads me to believe I should use the case for ground, but I'm not savvy with high voltage. Kinda funny for a guy who works in a nuclear power plant, huh?


    Could I use a 1MV Ultra-High voltage pulse generator for this?

    Like this:

    Hi, what kind of cable did you use for connecting the Neon sign transformer to the Jacob's ladder? Is it possible (and harmless enough) to use normal electric wire (like wire from electric devices)?

    1 reply

    I used regular wire, 14 gauge, I think.

    I made this with my kids, but modified the design somewhat. I used solid copper rods, threaded at the bottom, and bent so that the bottom gap could be adjusted with the nuts and washers. We used a 15KV transformer, and needed a 9mm gap at the bottom for the spark to start.