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(From: Kenny Greenberg (firstname.lastname@example.org).)
While it is true that warm air pushes the arc up the ladder, there is also the typical 'high leakage' or reactance curve of the transformer contributing to the effect. The transformer will happily arc across the bottom as long as Paschen's Law will allow. Once this arc is struck the current in the arc will actually increase to the transformer's preset limit. The heat is also creating higher resistance.
Normally the transformer would try choke the voltage down as current increased. But just above the arc exists a path that the transformer can easily maintain and which in fact will lower its current. Voila.
At the top of course we are not only at the upper limit of the transformer but it is also where the current is very low and so all the fun breaks apart only to reignite down below.
A very interesting variant existed in the 1930's which used separate horizontal electrodes at various points along one side instead of a continuous vertical line. Each electrode is attached to a separate neon unit. They are tied together and return to the HV transformer. The convection current was optimized by placing this in a housing with vents at the bottom and top. The result is an animator with no moving parts.
The downside is the nitric acid that gets produced so keep it away from things that may be corrode.
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!
Electrical discharges in air are also a producer of ozone which may be a health hazard. See the section: About Ozone Production. They also can produce significant Radio Frequency Interference (RFI) so the FCC may come calling if you run the thing for an extended period of time.
You will need 12 to 15 kVAC 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.
Typical ratings: 12,000 to 15,000 VRMS current limited to 20 to 30 mA. Larger ones are available - up to 60 mA or even 120 mA - but this is not required. More information on neon sign transformers can be found in the chapter: The Home-Built Laser Assembly and Power Supply of Sam's Laser FAQ.
Typical ratings: 8,000 to 10,000 VRMS current limited to 10 to 25 mA.
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 that 50/60 Hz buzz associated with those classic sci-fi movies! Perhaps, more of a starship engine sound. :)
Having said that, see the section: Notes on Really BIG Jacob's Ladders for some more information.
___ 1-3 inch gap or more at top. ^ \ / 2-3 feet | \ / or more | \ / 1/4 inch gap at bottom. _v_ / \ +----X-' '-X----+ X is mount attachment. | Insulated, | HV | non-flammable | +----+ mounting base | ||( | Hot o---+ ||( | )||( Luminous tube | HV insulated wire )||( 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!
You know how critical the gap between the electrodes at the bottom of a Jacob's ladder can be. Too wide and the arc won't strike, and too narrow and it won't make it all the way to the top.
Because I live in the UK I'm more or less saddled with a 10 kV limit on the maximum neon transformer available. Since the lower voltage makes the gap even more critical, I designed a slight enhancement that works really well.
It's simply a third electrode placed between the strike gap at the bottom of the vee. It is connected to either one of the main electrodes via two 1M ohm high voltage resistors.
When an arc should occur, the following happens...
I've never seen anyone use this on a Jacob's ladder before so what do I call it? How about the Gabriel electrode (in keeping with the biblical theme).
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.
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.
Snock's High Voltage Page has a number of articles and links relating to large Jacob's Ladders and other high voltage projects.
One of those old(bad) movies had a huge Jacob's 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)
(From: Pamela Hughes (email@example.com).)
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).
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