How to make a Jacob's Ladder


[Document Version: 1.27] [Last Updated: 05/25/1998]

1. About the Author & Copyright

How to make a Jacob's Ladder

Author: Samuel M. Goldwasser
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Copyright (c) 1994, 1995, 1996, 1997, 1998
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

  1. This notice is included in its entirety at the beginning.
  2. There is no charge except to cover the costs of copying.

2. Introduction

A Jacob's Ladder is the type of high voltage display seen in many old (and
usually bad) sci-fi movies.  Jacob's Ladder come in all shapes, styles, and
sizes.  Here is info on a common type that is easy to construct with readily
available parts.  However, read the section: "SAFETY" before attempting to
power any high voltage project of this type.

3. Basic components

There are only two major parts to a basic Jacob's Ladder: a high voltage
power source and a pair of wires or rods arranged in a narrow V configuration
on an insulated and fire proof support.

You will need 12 to 15 KV AC at 20 to 30 mA.  However, the exact values are
not at all critical.  A neon sign (luminous tube) transformer is the usual
source for this power though an oil burner ignition transformer will work in
a pinch (some say better and cheaper) or you could build an inverter type
power supply.

* Luminous tube transformers can be obtained used from sign shops or demolition
  companies.  The cost will be anywhere from free to $50 or more depending on
  size and condition and whether the seller has a use or other buyers for this
  sort of equipment.

  Typical ratings: 12,000 to 15,000 VRMS current limited to 20 to 30 mA.

* Oil burner ignition transformers can be removed from discarded oil burners.
  These will probably be free for the asking at your local HVAC company.
  However, you will likely have to disassemble the burner assembly and properly
  dispose of the unwanted parts as part of the deal.

  Typical ratings: 8,000 to 10,000 VRMS current limited to 10 to 25 mA.

* Both neon signs and oil burner ignitions may be powered by solid state
  inverters in some cases.  These are likely suitable as well - and higher

  Inverter voltage and current ratings will be similar to their transformer
  counterparts, above.  However, since the frequency of operation is in the
  10s of KHz range instead of 50/60 Hz, behavior for the Jacob's Ladder will
  differ somewhat.  For example, if they run on filtered DC (internally),
  there will be none of the 50/60 Hz buzz associated with those classic
  sci-fi movies!

* A small Jacob's Ladder can be powered from the types of high voltage
  inverters described in Various Schematics and Diagrams.
  However, since these only produce a couple of mA, the result will not be
  quite as spectacular.

* I DO NOT recommend the following: microwave oven transformers, utility pole
  transformers, and 100 KVA substation transformers :-).  See the section:
  "Why you don't want to use microwave oven transformers for a Jacob's Ladder"
  for additional comments if you are not yet convinced.

  - Microwave oven transformers 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 can supply AMPS of current which
    is an instantly lethal situation, are not current limited, and do not
    have the required insulation for such operation.

  - Utility pole and substation transformers.  Aside from requiring a fork
    lift or 10 ton crane to move, I don't think the power company would be
    happy if one of these were to disappear one night :-).  It goes without
    saying that these would even be overkill (no pun...) for the State
    electric chair.

  Having said that, see the section: "Notes on really BIG Jacob's Ladders" for
  some more information.

4. Construction

Take a pair of thick wires - the steel wire from old metal coat hangers works
pretty well - straighten them out and mount them with a gap of about 1/4
inch at the bottom and 1-3 inches at the top.  Of course, all on an insulated
non-flammable material!  Connect the high tension output of the transformer
to the two wires and you should be all set.  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 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.

                            ___   1-3 inch gap or more at top.
                             ^  \     /
                    2-3 feet |   \   /
                     or more |    \ / 1/4 inch gap at bottom.
                            _v_   / \
                          +------'   '------+
                          |   Insulated,    |       
                      HV  |  non-flammable  |
                     +----+  mounting base  |
                  ||(                       |
        Hot o---+ ||(                       |
                 )||(       Luminous tube   |
                 )||( CT     transformer    |
   AC Line       )|| +---+   (12 KV, 30 mA, |
                 )||(    |   typical)       |
                 )||(    |                  |
    Neutral o---+ ||(    |                  |
                  ||( HV |                  |
             Case |  +----------------------+
                  |      |
Safety Ground o---+------+   

IMPORTANT SAFETY NOTE: Essential line fuse, power switch, and power indicator
lamp not shown.  Centertap (case) MUST be connected to Safety (earth) Ground!!

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!

5. Why you don't want to use microwave oven transformers for a Jacob's Ladder

Using multiple high voltage transformers from microwave ovens to construct a
Jacob's Ladder is a very bad idea for several reasons:

* Microwave oven transformers are absolutely positively lethal if you or
  anyone else gets too close, AMPS available - especially if you are going to
  put them in series as would be needed to obtain adequate voltage.  You might
  as well tap directly off of your utility's substation power bus.

* Unlike luminous tube (neon sign) or oil burner ignition transformers, they
  are NOT current limited so some other means of preventing your breakers (or
  your utility's substation breakers) from popping like cherry bombs will be

* The most you will get out of a microwave oven transformer is about 2 to 2.5
  KVAC.  This is way to little for a decent Jacob's Ladder and putting them
  in parallel will not help.  Therefore, 4 or 5 in series would be needed to
  obtain adequate voltage.

* Their insulation is not rated for series operation so one or more of the
  windings will see excessive voltage.  There will likely be fireworks - and
  not where you want them.

I bet you are also going to try to run them on 220 VAC to double the output
voltage as well, huh??

Similar comments apply to the use of utility pole or substation transformers.

A word to the wise: at some point, bigger is just stupid.  Sorry.

6. Notes on really BIG Jacob's Ladders

(From: Steve Roberts (

Microwave oven transformers make excellent Jacob's Ladders and Tesla Coil
drivers with a properly chosen series cap in the primary for power factor
correction.  One of the major problems with the hollow "E" core current
limited neon sign and furnace ignition transformers is that they do not source
enough current to maintain a plasma on larger Jacob's Ladders, and the current
limiting makes them lousy as pulsed laser drivers as well.

However, the high current from a microwave oven transformer will source a 10"
inch arc in nitrogen/oxygen mixtures once it has been started at about 1/2".
There are several European web sites that show back to back microwave oven
transformer powered ladders.  A key problem is that you have to mount the
transformers on Lucite or Lexan because the center tap is grounded on the
secondary side. For a advanced safety conscious experimenter it is not a
problem to use these, provided you use either 1" diameter rods of copper or
carbon rods for the ladder rails. While I would not approve of just anybody
playing with these either, it can be done and produces spectacular results, if
you don't mind the electric meter spinning at Mach 1!

They also make nice stable cores for high current low voltage transformers
when rewound, I use them to power 25 amp 3.2 volt laser tube filaments all the
time with out blowing breakers. 

The following site has a number of articles and links relating to large
Jacob's Ladders and other high voltage projects:


  • 7. And now for the audio feed

    (From: Norman E. Litsche (
    One of those old(bad) movies had a huge Jacobs Ladder inside a large
    transparent (glass, I assume - Plexiglass wasn't around then) hollow column
    resonant at 60 Hz.  Unbelievable sound!  Always wanted to build one like this
    but never had the time, the big resonant column or the really huge neon sign
    transformer that would have been needed.
    (Assuming 1,100 feet per second for the speed of sound in air, a column closed
    at one end would be a 1/4 wavelength resonator resulting in an actual height
    of about 4.6 feet for a 60 Hz fundamental. --- sam)

    8. About ozone production

    (From: Pamela Hughes (
    The arc is a plasma of hot ionized gas.  Molecules like O2 are broken down to
    the atomic level and ionized.  when these ions collide with the surrounding
    air, they cause chemical reactions... the O can combine with nitrogen and form
    small amounts of nitrogen oxides, and with O2 to form ozone (O3).  However,
    the high temps in an arc also tend to destroy these molecules too so you'd
    probably only produce trace amounts if it weren't for the UV given off by the
    arc.  Ultraviolet seems to be the main mechanism for producing O3 as it can
    ionize in the air far enough from the arc that it will be cool enough for
    ozone to exist (a spark gives off UV and ionizes the air around it) A glow
    discharge is better at generating ozone than an arc though, since it maximizes
    the UV and the pressures and temps are much lower (i.e., put a conductive
    coating on the outside of a glass tube and a wire down the center of it, then
    apply enough voltage to produce a glow discharge inside the tube as you pump
    oxygen at low pressure through the tube.  Shortwave UV lamps will produce it
    too (they use these as sterilizers in dairy barns).

    9. SAFETY

    CAUTION: See Safety Guidelines for High Voltage and/or Line Powered Equipment
    before firing up this circuit!
    Make sure that no one can come in contact with this - particularly curious
    onlookers.  Separating the potential victims from any possible contact
    with the high voltage is really the only foolproof way of protecting
    against fools or the unaware - and you from a lawsuit.  People not
    familiar with high voltage phenomena (or aware only through grade-C
    sci-fi movies) can be incredibly naive.
    A GFCI (Ground Fault Circuit Interrupter) is of no use in protecting against
    HV contact since the secondary of a neon sign transformer is isolated from the
    line but its centertap is usually connected to the case - which should be
    grounded.  However, a GFCI would be a good idea in any case when you are
    working with line connected equipment.
    12,000 volts will jump approximately anywhere from 3/8 to 3/4 inch in dry air,
    with sharp points and edges generally but maybe not quite always favoring
    longer distances.  This distance occaisionally varies unpredictably with
    humidity.  Don't forget that 12,000 VAC is approximately 17,000 V peak.  Neon
    sign transformers have current limited outputs - 30 mA is typical - but that
    is still highly dangerous - lethal under the wrong conditions.
    You can build a small Jacob's Ladder using a high voltage transformer
    of lower capacity or a DC-AC inverter using a TV flyback transformer.
    While these would be less dangerous, there is little room for carelessness
    when working with any type of high voltage device.  Even if there is no
    resistive path, the stray capacitance can permit enough AC current to flow
    to give you a painful experience!
    * Do not work alone.
    * Always keep one hand in your pocket when anywhere around a powered
      line-connected or high voltage system.
    * Wear rubber bottom shoes or sneakers.
    * Don't wear any jewelry or other articles that could accidentally contact
      circuitry and conduct current.
    * Use an isolation transformer if there is any chance of contacting line
      connected circuits.
    * Don't make adjustments with power on.
    * Finally, never assume anything without checking it out for yourself!
      Don't take shortcuts!

    Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server [Copyright] [Disclaimer]