From: Richard Sullivan FRPS (richsul@earthlink.net)
Date: 10/24/01-09:32:59 AM Z
Sam,
No problem with getting the rods. Apparently they are the same as sold in
welding stores and are still used in movie projectors in outlying areas.
Here is an interesting article I found. It looked like it was worth the
band with to send to the list:
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.
At 08:06 AM 10/24/2001 -0700, you wrote:
>Thank you for your reply
>This thing is pretty old. I think it was made for
>making litho plates. I have a handful of rods, any
>idea how long they last? I can vent it without to much
>trouble.
>I think I am leaning towards just leaving it and using
>it as is.
>Again, thank you for your response.
>
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