*                    VARIOUS NOTES ON CARBON ARCS                      *
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  *                       **** Version 1.03 ****                         *
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  *                         Copyright (C) 1996                           *
  *                        Samuel M. Goldwasser                          *
  *                           Don Klipstein                              *
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  *                   Corrections or suggestions to:                     *
  *            sam@stdavids.picker.com or don@donklipstein.com           *
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Carbon arc basics:

The carbon arc is quite an old technology predating the invention of the incandescent lamp. For a while, it was even considered as an alternative to it - just think, we could be reading by carbon arc light!

Until the advent of appropriate high intensity gas discharge lamps, the motion picture projectors in movie theaters used carbon arc lamps. Your neighborhood theater may still use these. Sometimes the rods would need to be changed in the middle of a reel and the screen would go dark for a minute or two.

The technology is quite simple. A pair of carbon rod electrodes are connected to a current limited source of power - 115 V AC or DC in series with a 1500 W space heater, for example. They are mounted on a well insulated, fire proof structure which allows the distance between the rods to be controlled.

Carbon rods can be extracted from most flashlight batteries, preferably c-cells for currents around 10-15 amps or so. Please note that flashlight batteries are filled with various forms of gunk, which may be corrosive to some extent or another. The cheapest types of batteries such as Radio Shack's red ones or Eveready "Classic" (with a cat jumping through a numeral 9) have carbon rods, and only slightly corrosive gunk. The gunk must be cleaned off your hands, clothing, carbon rods, anything else, etc. One substance in this stuff is manganese dioxide, which can corrode some metals when wet, and may increase the flammability of a few combustible substances (especially some combustible metals) if dry or slightly damp.

Please note that manganese dioxide makes smudgy stains that are hard to remove from anything even slightly porous. In the event you get it on anything washable, it can probably be removed by some acids, such as moderately diluted sulfuric or hydrochloric acid. Remove traces of the acid afterwards by rinsing with massive quantities of water or wet baking soda. Hydrochloric acid is sold by a few hardware and construction supply stores as "muriatic" acid.

Carbon rods made for carbon arcs are available at some welding supply stores. These carbon rods often have a copper jacket to improve conductivity. The copper melts away from the tips of the carbon rods, exposing a short portion of the carbon rod.

Do not connect an arc directly to a power source. Something must be in series with it to limit current. Most arcs have a slightly nasty characteristic, becoming greatly more conductive as they get hotter. Without current limiting, an arc will draw current largely limited by your household wiring, and might even cause nasty effects of many kinds before your fuse blows/breaker pops.

For a do-it-yourself carbon arc, current limiting is usually done with some sort of high-wattage heating device such as a space heater. This is used as a resistor.

To start the arc, the power is turned on with the carbon rods separated. They are then brought together until they touch and gradually separated until a nice steady (and extremely bright!) arc is formed.

The carbon arc itself is fairly bright, but the tips of the carbon rods are usually much brighter. The tips of the carbon rods get heated up to a temperature usually near 3600 degrees Celsius, or approx. 6500 degrees Fahrenheit. This is near the melting point of carbon. At this temperature, the carbon tips are brighter than halogen lamp filaments of comparable size.

If you heat the carbon rod tips to the melting point of carbon, you will probably *not* get puddles or even dripping drops of molten carbon, since molten carbon evaporates VERY easily.

In addition to light, there is usually generation of noxious smoke, carbon dioxide, possibly significant amounts of carbon monoxide, oxides of nitrogen, - and possibly small quantities of Bucky-Balls (Buckmisterfullerine, C60) as well.

Although the carbon monoxide emissions are usually minimal, they could be much greater if the arc is enclosed in a partially closed container that lets just the wrong amount of oxygen interact with the hot carbon and/or carbon vapor. Because of the possibility of carbon monoxide as well as other noxious gases and fumes, it is recommended not to operate a carbon arc indoors for more than a few seconds unless ventillation is very good.

The light emission is broad spectrum including IR and UV (often in hazardous quantities if not filtered). The UV content contains significant UV-B and some UV-C (shortwave UV) which is hazardous to skin and eyes. Ordinary glass stops these, but plenty of UV-A (longwave UV) gets through glass, and this may be hazardous to eyes at high intensities.

Please note that the white-hot carbon rod tips are hazardous to look at, even if all UV and IR is removed. They are several times brighter than a halogen lamp filament of similar size. If you use goggles made for acetylene welding, then you can probably safely look at a carbon arc for a few seconds. These give some light attenuation, along with greater attenuation of UV and IR. To safely look at such arcs for prolonged periods of time, an appropriate arc-welding face mask is recommended. Acetylene goggles let through too much light and possibly too much UV to stare at the arc for much more than several seconds. You also need to protect all exposed skin from shortwave and mediumwave UV if you will encounter more than casual exposure to arc radiation.

As the the carbon electrodes wear, they must be moved to maintain the distance between them constant. Actual carbon arc equipment used a feedback mechanism which monitored the current and adjusted rod position to keep it constant (the current would decrease as the arc length increased). The light output from such devices was remarkably constant.

DC Carbon Arcs

Arcs are not electrically symmetric. If an arc is powered by DC, one end is usually different from the other. In DC welding arcs, the greatest concentration of heat usually occurs at the negative end due to the "cathode fall", the voltage drop involved in getting electrons from metal into the gas or vapor. In addition, since some of the metal vapor is in the form of positive ions, metal tends to be depleted from the positive electrode and some of this may even be deposited on the negative electrode.

DC carbon arcs are similarly assymetric. The positive electrode is depleted more rapidly than the negative one. A crater usually forms in the tip of the positive electrode.
Unlike most arcs involving metals, the cathode fall in a decent carbon arc seems to be minimal. The positive electrode makes more light than the negative one in most cases. In carbon arc searchlights and projection systems, a DC arc is usually used so that most of the light is emitted from only one spot, the crater in the tip of the positive electrode.

If you find the negative electrode to be as bright as the positive one or brighter, you may have too little current for the size of your carbon rods. You may want thinner carbon rods, or more likely, more current. Low current reduces cathode heating, which makes electron emission more difficult. This increases the cathode fall, which results in cathode heating nearly as great as that of higher currents.

Carbon arcs for fun and danger:

Note: these sorts of experiments are particularly hazardous - duplicate at your own risk. There is danger from AC line connected high power, risk of setting fire to something including yourself, and risk from substantial emission of shortwave and mediumwave UV which is bad for skin and eyes, and the risk of noxious fumes and/or gases.

(From: Arnold Pomerance (pomeranc@goldsword.com)).

Here is some information about carbon arcs that may be helpful to your story.

Back in junior high school (1962) I built a small open-top carbon arc furnace as a science project whose topic was "How are metals melted?". It attracted a lot of attention at the school science fair, since it produced a *lot* more light than heat! :)

I recently came across parts of it while cleaning out my parents' house. Looking back on that project, it was incredibly dangerous, and I am very lucky not to have been injured.

Anyway, I managed to drill two 1/2 inch holes across from each other, about halfway up the sides of an ordinary clay flower pot whose top was about 6 inches in diameter. With black furnace cement I glued its base to a piece of firebrick for stability. The firebrick was held loosely captive by four small wooden blocks screwed onto a piece of plywood about two feet square. On both sides of it I mounted some vertical 2 by 4 wood uprights, with 1/2 inch holes drilled in them that were lined up with the holes in the flower pot. The uprights were about 6 inches out from the pot on each side.

For electrode holders I used foot-long pieces of 3/8 inch (outside diameter) copper tubing. For safety (ha!) I press fitted them tightly into lengthwise 3/8 inch holes drilled about 2 inches deep in 4 inch pieces of wooden dowels (i.e., cut up broomstick handle), to act as protective handles. Well, they were protective in the sense of being relatively nonconductive for both heat and electricity! :) These electrode holders could be slid towards and away from each other, to meet in the center of the flower pot.

Power entered a small pull-handle fuse box (also mounted on the plywood, near the front) via a 120V 15A cord which could be plugged into any convenient outlet. From the fuse box, I connected one side to my parents' portable broiler (i.e., using it as a high-wattage 10A ballast resistor) which sat behind everything else, out of the way. I connected the other side of the broiler to one of the electrode holders. I completed the circuit by connecting from the other electrode holder back to the fuse box.

The wiring was flexible high-temperature cord, such as for a steam iron, with both conductors twisted together at both ends to double its current carrying capacity (and because I only needed one conductor) on each leg. I stripped about 1.5 inches of the stranded copper wire and fastened it to the copper tubing by looping it around the tubing and snugging it up to itself with an ordinary split-bolt lug. (Yes, it needed to be re-snugged every once in a while; I never did figure out a better way at the time; I suppose nowadays I would use a screw into the tubing...) At the broiler, I simply wrapped the copper strands around the pins that normally would accept the socket end of an appliance cord. :)

I was *very* aware of all the exposed 120V connections! The pull handle on the little fuse box was a very reassuring safety device, and I never hesitated to pull it to disconnect the power whenever anything needed to be adjusted, or connections tightened, or electrodes changed, or experiments set up...

Finding a supply of solid carbon electrodes was no problem: I merely hacksawed open some used D cell batteries and pulled their carbon rods out! Then of course I had to scrape all that yucky acid gunk off them with a pocket knife! :-o But they were conveniently about 1/4 inch in diameter, and could be pressed (with difficulty) into the copper tubing, whose ends had been flared somewhat with a screwdriver.

When setting up the furnace, I would scoop about an inch or two of vermiculite granules into the bottom of the flower pot as a heat resistant blanket to catch any hot objects (including loose carbon rods!) that might fall from the arc region.

To operate: Separate the carbons about a half inch. Turn on the power. Put on a welding hood. (Actually, I used ordinary sunglasses, and suffered ultraviolet burns of the eyes as a result of inadequate protection; for several days afterwards my eyes felt as if they had gravel in them!) :( Move the electrodes towards each other until they touch. Loud hummmm! Slowly pull them apart about 1/4 inch to create the arc. Tremendous blue-white light! Loud, raspy buzzing at about 120 Hz! Steady wisps of burnt carbon-smelling smoke! (Mostly from vaporizing carbon, but for the first few minutes from remnants of the battery chemicals also.) 8-o

If left alone, the arc was stable for only a minute or two, because the carbons gradually eroded, making the arc longer and longer until it was too long to sustain itself. So I had to gently adjust the spacing frequently. I remember noting that a long arc was considerably brighter and noisier than a short arc, so I could sort of tell what the spacing was without looking at it. As if there were any way I *COULD* look at it--my measurement technique consisted of killing the power and then looking! :)

For my experiments I cut small tin cans into winged shapes to make little pots (crucibles, kind of) for melting things like lead tire weights and other metal scraps, suspended about an inch over the arc by hanging (with their wings) from the top of the flower pot. Water in such a pot would boil in a minute or so. Of course the steel pots would oxidize and eventually burn through. To demonstrate melting a scrap of solid copper wire I had to forego using a pot and instead pass the wire directly through the center of the arc while holding it with long nose pliers (and wearing thick gloves!) :-o

Years later I read that carbon arcs tend to produce large quantities of carbon monoxide gas, so professional carbon arc lights have vent pipes to carry it away. :( I hadn't known that, or even guessed. Like I say, I was lucky!

I also read that professional carbon arc lights use DC instead of AC for several reasons: A DC arc is continuous and reliable, whereas an AC arc must re-strike itself many times each second and tends to blow out more often. There is less buzzing with DC than with AC. With DC, there is just one spot of bright light (anode? cathode?) to focus with the lenses, instead of two spots. (Apparently the electrode tip is a much brighter source of light than the arc itself.)

On the other hand, one of the DC electrodes will erode considerably faster than the other, as opposed to equal erosion with AC. Also, a DC arc requires a steady (preferably regulated) source of DC, whereas an AC arc can get by with just a ballast resistor. Sorry, I never measured the current or voltage, so I don't know what the effective resistance of my arc was.

Brighter Illuminating Arcs

1) Cored Carbons

The arc can be enhanced by using hollow rods that are stuffed with other substances whose vapors glow brightly in arcs. Various metal salts are usually used for this. Ordinary sodium chloride even works for this, resulting in large quantities of orange-yellow light. Strontium compounds will produce a more red or pink color.

Please note that the salt vapors will condense into smoky fumes when they leave the arc. Depending on the substance used, these fumes may be hazardous to breathe and/or cause corrosion problems if they settle on metal parts, especially if combined with moisture or even humidity afterwards.

2) Magnetite Arc

Sometime way back when the high-pressure mercury lamp was not yet used for street lighting, arc lamps were used in a few locations for this purpose. Charles Steinmetz was able to improve on the carbon arc by using magnetite (an iron ore mineral) instead of carbon. The magnetite released iron vapor into the arc. Like many other metal vapors, iron vapor results in a brilliant arc with a characteristic color. Iron arcs are typically a purplish shade of blue-white.

Please read Don Klipstein's Disclaimers.

--end V. 1.03 (carbon arcs)

Some arc metal melting experiments by Klaas ("C++ freak"). (Ditzhuyzen.Klaas.van@uniface.nl) Caution, such experimentation can be hazardous.

A bit of carbon arc lamp history including a few details and some photos of actual historic arc lamps. (from the Insitiute of Electrical Engineers in the UK)