This is a sketch of the Jacob's Ladder found in our lobby.
If you haven’t already, check out one of the newest exhibits out on the floor (or more accurately, in the entrance): Jacob’s Ladder. This sci-fi (think: Gene Wilder’s laboratory in Young Frankenstein) inspired exhibit takes its name from the biblical story describing Jacob’s vision to build a ladder to heaven. In the case of our exhibit, though, the rungs of the ladder are made up of glowing arcs of ionized air and electrons. The entire apparatus consists of a copper electrode running up the center of a giant cylinder, with smaller wires running up and away from the central electrode. A strong voltage (around 30,000 V) is applied to the copper electrode running up the center of the apparatus that causes an intense spark. The resulting spark is caused by an attempt of the electrons in the current to complete a circuit by “jumping” from this copper electrode to a metal wire. This spark ionizes the surrounding air molecules, thus dramatically reducing the resistance from the molecules and creating a conductive arc of ionized air called plasma. The resulting drop of resistance from the air causes the voltage between the two electrodes to drop dramatically, and electric current flows freely from the copper electrode to the surrounding wires that run up and away from the center.
The ionized air forms a brightly glowing arc due to fluorescence caused by excitation and subsequent relaxation of electrons in the air molecules, a process that releases photons and produces light. Electrons in the flowing current collide with air molecules, causing the electrons within these ionized molecules to become more energetic, or excited. When these electrons eventually relax into their original energy levels, a particle of light called a photon is released by the air ion. Because this process depends on the emitted photon having less energy that the original colliding electron, the difference in energy is made up in heat absorbed by the ion, which causes the plasma that forms the plasma arc to become warmer and rise relative to the surrounding non-ionized air. As the ionized air rises and the electrodes pull away from each other, the stability of the arc diminishes until the distance between the copper tube and the wires is too great and the arc snaps. Because the current at this point stops flowing, the voltage at the bottom of the apparatus shoots up again, leading to another spark, which starts the process over again.
The cracking, popping and snapping noises are heard coming from the cylinder are caused by the forceful stripping of electrons from the air molecules inside the chamber, which, under normal conditions, are actually quite thermodynamically and chemically stable. As electrons are removed, the air molecules become charged; the ionized air (plasma) rises and sends out a compression wave that is interpreted by the listener’s ear as sound. The conductive plasma rises as the air gets hotter, and this shift causes the pitch of the sound to change as the arc climbs higher. This electrostatic sound gives the Jacob’s Ladder it’s eerie, mad scientist feel.
Fluorescence is a unique property for different molecules; the wavelengths of emitted photons are dependent upon both the type of molecule fluorescing and the energy of the colliding electron. Depending on the gas present in the container, the color of the glowing arc will be different. Because air is a mixture of 78.08% nitrogen gas (N2), 20.95% oxygen gas (O2), 0.93% argon (Ar) and 0.038% carbon dioxide (CO2) with trace amounts of other gasses, the color of the fluorescent arc is almost yellow-white. Were the container to be filled with only CO2, the fluorescence of the molecules would produce blue photons; nitrogen gas would glow with a pink color, and the noble gases would be capable of producing very bright arcs at a variety of wavelengths. Because electrons possess the properties of both particles and waves (a property called “duality”), they can travel without a medium present, similar to light. Were the chamber to be evacuated and be free from gas, the electrons that form the electrical current would find themselves moving about the cylinder without a target to ionize. There would be freely-moving electrons filling the chamber, but there would be no electrostatic noise or bright glowing arc because each of these features of the Jacob’s Ladder are dependent on the present of a gas to fluoresce and form a plasma.
Lightning, a natural phenomenon still not entirely understood, is thought to work in a similar manner. Charge separations in clouds are thought to create a “spark gap” like the one found between the copper electrode and the surrounding wires that ionizes surrounding air, making it an excellent conductor for the large amounts of electricity that becomes a lightning strike. This type of technology has also been employed in combustion engines in the form of a spark plug. Spark plugs use a spark gap to create an electrical discharge that, in the case of car engines, ignites compressed gasoline vapors rather than air. Jacob’s Ladder-type devices can also be found in industry to protect electrical equipment. Anywhere charge separations in electrical components that are suspected of creating a spark gap are present, conductive wires are installed in a manner similar to the layout of our exhibit in order to catch the spark before it reaches sensitive instruments, allowing the current to rise and dissipate. This prevents a sudden surge of electrical current from causing irreparable damage.
With all of these applications, the technology employed by the Jacob’s Ladder is bound to show up elsewhere within the rest of the Ruben H. Fleet Science Center. During your next visit, take some time to explore the other exhibits and see if you can’t find these possible connections. Are there other exhibits on the floor that utilize electricity in interesting and visually stunning ways? What about sound? Explore your own surroundings as well to see if you can identify phenomena similar to fluorescence and electrostatic sound. And the next time you find yourself watching late-night cable and you flip to a B-rated sci-fi flick, take a look at the set décor. Odds are you’ll find a Jacob’s Ladder peeking out from the shadows in the background!
References:
Goldwasser, Samuel M. “Jacob’s Ladder (Climbing Arc) Construction.” 19 Apr. 2006. 29 Mar. 2009. http://www.repairfaq.org/sam/jacobs.htm
Lowke, J. J. “Theory of Electrical Breakdown in Air- The Role of Metastable Oxygen Molecules.” Journal of Physics D: Applied Physics 25 (1992): 202-10.
“Spark Gap.” 12 Aug. 2003. 29 Mar. 2009. http://en.wikipedia.org/wiki/Spark_gap
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