Introduction: How to Build a 12 KV Jacob's Ladder
*Figure numbers for pictures are shown once you open up the picture in the top left corner
A Jacob's Ladder is one of the easiest fun displays to make using high voltages, as seen in old sci-fi & horror films. A small arc starts at the bottom, and slowly rises into a snaking, flame-like buzzing arc. All you need is a current-limited high voltage AC supply from a suitable transformer (neon sign transformers work quite well), and some copper wiring.
(1) 12kV Neon Sign Transformer (NST) (eBay)
(1) 300V Toggle Switch (https://www.amazon.com/gp/product/B01IYKSLCG/)
(20 m) 14 AWG 30kV High Voltage Wiring (https://www.amazon.com/gp/product/B01IX3B8YC/)
(30 ft) 16-2 AWG Generic Two Line Wiring (Home Depot)
(30 ft) 14 AWG Three-Pronged Extension Cord (Home Depot)
Heat Shrink Wire Connector Kit (https://www.amazon.com/gp/product/B06VW8LJFN/)
(1) Roll of Bare Copper Wire, Bright, 12 AWG, 0.08" Diameter, 50' Length (https://www.amazon.com/gp/product/B0051X4EJ4/)
(1) 3/4" x 2ft x 4ft Plywood Board (Home Depot)
(1) bag of #10 x 1/2" Hex Head Zinc Sheet Metal Screws (Home Depot)
3D Printed Parts:
Various 3D Printed Parts Attached
Print using: a perimeter of 4, infill of 15%, layer height of 0.2mm. This will make the prints the strongest.
Soldering iron and solder
Drill and drill bits
Wire strippers, wire cutters, and pliers, electrical tape, etc.
Measuring tape, ruler, etc.
Step 1: Safety!
Jacob's Ladders are extremely dangerous!
High voltage alternating current is not something to scoff at!
Jacob's Ladders have killed people!
A 15 year-old in Ohio was killed while trying to make a Jacob's Ladder at home. (See  in "Sources" Step)
Make safety a top priority while working on a Jacob's Ladder!
The NST is especially dangerous because it supplies several thousand volts, and you'll be working in close proximity to it. It's easy to accidentally leave it turned on. Except for a very quiet humming, there's no indication it's turned on.
Step 2: How Does It Work?
When the Jacob’s Ladder is turned on, a potential difference is produced between the two electrodes. Electrons in one electrode want to jump to the other electrode to balance out the charge.
We see a bright spark in the air when the electrons make the jump. This spark is comprised of ionized air, which was a much lower breakdown voltage than normal, unionized air. The breakdown voltage is the voltage at which a spark will occur.
This spark is very hot, so hot that it can be classified as a plasma. Remember that hot air rises, so the hot ionized air rises. The spark wants to jump through the path of least resistance. Since the ionized air has less resistance than the surrounding air, the spark will rise with the hot, ionized air to continue with the path of least resistance.
If tuned correctly, the bottom of the Jacob's Ladder will have a smaller gap that widens as the height increases.
As the spark rises, the resistance of the ionized air increases since the gap increases. When this resistance becomes greater than the resistance at the base of the Jacob's ladder where the air is not ionized, a new spark is created at the base since electrons seek the path of least resistance. (See Fig. 2)
Step 3: Power Supply (NST)
The power supply is a transformer made for neon signs, and they are the best option for a Jacob's Ladder.
There must be a current limiting source in a Jacob's Ladder. The NST has a built-in series inductance, which limits the current when they are shorted so there is no problem in using them for Jacob's Ladders. 
An example NST can be seen in Fig. 3 above. I purchased this one off of eBay. Input wiring of NST can be seen in Fig. 7.
- Around two feet away from the male end (the one with prongs sticking out) of the 14 AWG extension cord, carefully strip off a 6-inch section of wiring. Do NOT cut it.
- Cut ONLY the black wire. For the 16 AWG generic wiring, connect the one wire from one end to one of the black wires you just cut. Connect the other wire from the same end to the other black wire you just cut.
- By now, you should have the 16 AWG generic wiring leading away from the thicker extension cord. After confirming, solder all of the connections and wrap using electrical tape.
- Connect the free end of the 16 AWG generic wiring to the toggle switch. Solder the connection. Mount this toggle switch to the 3D printed toggle switch case (files attached)
- Using a multimeter, connect the "Hot" end (the one with the smaller prong and opening) from both the female and male ends of the thicker extension cord. Check that these two ends are connected whenever the toggle switch is on, and disconnected whenever the switch is off.
- Make a cut at the female end of the extension cord (the one with openings for a plug).
- Strip off a 6" section of wiring, and connect each of the individual smaller wires to a heat shrink wire connector. This will be used to connect to the NST, so make sure the wire connectors fit.
- The black wire ("hot") will connect to the "+" input terminal of the NST, and the white wire ("neutral") will connect to the other terminal. The green wire is the ground, which is connected to the unmarked separate terminal.
- There should be two output terminals on either ends of the NST, they are the same, so it doesn't matter which side is connected to what part.
- Check your work, and make sure you wrapped everything with electrical tape.
The NST is essentially two coils of wire that uses inductance to step up the voltage while lowering the current.
The NST shown in Fig. 3 above has an output voltage of 12 kV.
A good NST should be very heavy and only contain primary and secondary windings and an iron core. The output frequency should be the same as the input frequency (50 or 60 Hz).
NSTs have shunts that will limit the current if the output is shorted.
Step 4: Set-Up 1 Fail!
Originally I used 18" long, 1/4" copper tubing for the electrodes.
However, the tubing is unable to bend at sharp angles. As a result the spark stays at the initial gap of the Jacob's Ladder and doesn't rise.
Step 5: Final Setup
The final setup with two electrodes made using 14 AWG bare copper wire works quite well; however, one downside of using copper wiring is that the flexibility results in unstable electrodes that often disturbs the spark.
- Cut a piece of plywood board measuring 16" x 14" x 3/4". (See Fig. 11)
- Mark the center of the board. (See Fig. 12)
- 3 cm from the center of the board, lengthwise, drill two holes using a 1/16" drill bit. (See Fig. 12)
- Cut two 16" long pieces of 14 AWG bare copper wire.
- Strip 1" off the ends of two pieces of high voltage wiring.
- Secure the electrodes (bare copper wire) and high voltage wiring to the board using sheet metal screws in the holes drilled in Step 3. (See Fig. 13)
- Bend the electrodes to match Fig. 14. This way a spark will form and rise up along the electrodes.
Step 6: Colors!
You may have learned in Chemistry that burning different metal chlorides will produce different colors. By dissolving these chlorides in water and coating the two Jacob's Ladder electrodes with the aqueous solution, different colors can be produced (See Fig. 15)
When dissolving chemicals, be extremely careful as many metal chlorides are toxic. I chose to use LiCl, CuCl, CaCl2, and KCl since these were the easiest to obtain and produced unique colors. (See Vid. 1)