Jacob's Ladder

"...And he dreamed, and behold a ladder set up on the earth,
and the top of it reached to heaven: and behold the angels of God
ascending and descending on it..."
                                                                          Genesis 28:12

      The device known as Jacob's Ladder is a perennial favorite, and is often the first high-voltage project completed by experimenters; it is simple, easy to make, forgiving in plan and construction, and produces a unique effect.

        Though it bears no comparison to the glories witnessed by Jacob, nevertheless it must have greatly impressed those who first saw the leaping white fire rising on the rails.  In today's high-tech, pyrotechnic, laser and Tesla lightning effect environment Jacob's Ladder can seem passe; old news, relegated to a seldom-visited place in black and white horror film history.  Most people alive today have never seen one except in movies, but switch one on in front of them, and the response is always the same; they stare at the rising plasma in rapt fascination as if it is something entirely new.  This is why Jacob's Ladder refuses to die, constantly reappearing in home shops and garages across the country to amaze new generations.

What is Jacob's Ladder and how does it work?

        The device consists of a high voltage transformer which steps up the mains voltage to 5,000 to 20,000 volts or so.  The output terminals of the transformer are connected to two vertical metal wires or rails, spaced a distance apart depending on the voltage used - for a 15,000 volt neon sign transformer, 1-1/2 to 2-1/2 inches.  At the lower end, the rails are brought closer together to make a starting point for the arc.  At the top, they are flared apart.

        When the transformer is switched on, the high voltage jumps across the small gap at the lower end.  This arc heats the air in and around it.  The hot air rises, and because hot air is a better conductor of electricity,  the arc takes this path, rising with the warm air.  Once the arc is established it can stretch out for some distance before it breaks; the rails spread out just above the start to take advantage of this and maximize the arc size for the viewers' pleasure.  Why doesn't it immediately re-form at the lower, closer gap?  The hot arc path, which can be many times longer than the short gap at the bottom, has a much lower resistance and electrically appears much shorter to the current, which takes this path of least resistance. The arc travels up the rails, turbulence changing its aspect, until it reaches the wider area, stretching until it breaks. Now without its hot air path, it has no choice but to re-form at the lower gap and the process starts again.

        The actual current in milliamps or amps determines the appearance of the arc and how far it may stretch.  A high voltage with just a few milliamps or less appears as a thin violet-white stream and cannot be pulled out as far as a current of 30 - 60 milliamps, which appears as a grey-white flame about 3/8 of an inch thick up to 4" long.  A current of 240 mA to an amp or so is fatter, stretches much farther and generates enough heat and turbulence that it ripples and makes snapping noises much like fire.

        The arc rises at a rate influenced by how fast the hot air channel moves.  If the rails are enclosed in a suitable chamber or tube, a draft can be created which assists the arc up the rails quickly.  On the other hand, if the air flow is restricted the arc can be made to rise very slowly.  Limits on rail length are more practical than theoretical.

        The color of the arc can be affected or changed by chemicals or impurities on the rails or in the air. Dust, rust or dirt often produces a yellow color. An alcohol lamp containing some dissolved boric acid not only makes the alcohol flame greenish, the combustion product boric oxide rising into the arc produces a green color. Metal salts such as sodium chloride (table salt) applied to the rails can give color to the arc, but must be replenished from time to time.

        Sometimes a short Jacob's ladder known as a 'horn gap' is used on power distribution networks to break arcs that may form across terminal connections from surges, lightning strikes, et cetera.

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Copyright © 2000 by David McNamee. All Rights Reserved.
This part of this site last revised August 15, 2000.