Subject: [Q] Carbon-arc questions after experiment

From: C++ Freak <[email-masked]>
Date: 10 Jun 1996 07:22:40 GMT

I tried an electric arc using carbon rods from a 4.5 Volts battery (1/4" diameter)
which I put into holes in a chamotte brick with a hole in the center where the
arc would take place. The electrodes are put horizontally.
The purpose is melting metals with high melting point (> 1500 C).
I put the arc in series with a 1500 W electric heater as a ballast on the
220 V 50 Hz (default in Europe) mains. I had to be very aware of the live wires
and used well-insulated screwdrivers to move the electrodes.
Starting when they just touch each other resulted in a bright white light.
Then I *slowly* put them apart until 2 cm (3/4") after half a minute and the humming
noise got louder but the heater got lower. The arc was fairly stable, but putting
a nail in the arc disrupted it. Then I had to switch off power, 
and redo the whole ritual such as putting electrodes close etc.
But it got *very hot* and melted the chamotte easily.
Viewing with my spectroscope, it showed a rather continuous
spectrum with several emission lines superseded on it. When the arc got a little
unstable (made more noise and got brighter) the lines got stronger.
Looking directly into the arc was only possible with *two* pairs of goggles
over each other used normally for an oxyacetelene torch.

I have three questions:

- How can I feed metal into the arc without disrupting the arc so that it melts ?
- Can I use an inductive ballast rather than a energy-wasting resistance ballast
  like an electric heater ? Gas-discharge (based on the same principle) 
  lamps do use them.
- Why do iron electrodes not work (it only sparks slightly) ?
  
Thanks in advance.

Klaas

---

From: Roger Gibbs <[email-masked]>
Date: Mon, 10 Jun 1996 17:54:05 GMT

>I tried an electric arc using carbon rods from a 4.5 Volts battery (1/4" diameter)
snip
>over each other used normally for an oxyacetelene torch.
snip
>- Why do iron electrodes not work (it only sparks slightly) ?

1. If it were me, I would not use filters intended for oxyacetylene
welding to view an electric arc.  Gas welding produces relatively
low levels of UV compared to arc welding.  I do not know, but I
suspect, that the filtering characteristics of arc welding filters
are such that a single arc welding lens will filter more of the UV
than two gas lens.  Again, unless I had specific knowledge 
regarding the filtering characteristics, I would not take the chance.

2. Regarding iron electrodes not sustaining an arc: arc welding rods
are coated for at least 3 reasons: 
     1. As the heat melts the coating, a relatively inert gas in 
					formed which shields the molten/hat metals
     2. The coating condenses to form a protective slag over the weld
     3. The vaporized coating releases ions which support the flow
     of electrons and play a major role in sustaining the arc.
I am guessing that carbon ions are released into the arc plasma and
sustain the arc when you use carbon electrodes.  Iron, by itself,
 does not do this well.

-Roger

---

From: Dan Mills <[email-masked]>
Date: Mon, 10 Jun 96 19:22:00 GMT

In article <[email-masked]> [email-masked] "C++ Freak" writes:

[Chop]

>
>I have three questions:
>
>- How can I feed metal into the arc without disrupting the arc so that it melts

Try using the metal as one side of the arc, I would suggest making the arc 
vertical with the electrode lowered onto the metal. 

>- Can I use an inductive ballast rather than a energy-wasting resistance ballast>  like an electric heater ? Gas-discharge (based on the same principle) 
>  lamps do use them.

Yes, but the power factor will be very poor. The carbons normally used in 
arc lamps are copper clad to reduce the voltage drop. I would suggest trying 
to get some of these. I also think that you really need more current to get 
this to work, If you can find one a old penthouse rectifier would work well,
these were used to power the carbon arc lamphouses in film projection until
the advent of xenon lighting. 

>- Why do iron electrodes not work (it only sparks slightly) ?

The carbon is consumed by the arc so as to form a conductive plasma.

-- 
********************************************************
* And on the first day the lord said...... *           *
*......Lx1, Go! and there was light!       * Dan Mills *
********************************************************
DMX-512 FAQ : ftp.webcom.com/pub1/lightsrc/ftp/dmx512fq.txt

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From: [email-masked] (Jonathan M. Elson)
Date: 11 Jun 1996 06:19:08 GMT

C++ Freak ([email-masked]) wrote:
: I tried an electric arc using carbon rods from a 4.5 Volts battery (1/4" diameter)
<snip>
: I have three questions:

: - How can I feed metal into the arc without disrupting the arc so that it
:  melts ?

Probably you need to pre-heat the metal.  Try keeping the carbons closer
than the max distance.  Then introduce the metal rod very slowly at first,
so the radiant and convective heat transfer can pre-heat the rod.  then,
feed the rod into the arc.

: - Can I use an inductive ballast rather than a energy-wasting resistance
:   ballast like an electric heater ? Gas-discharge (based on the same
:   principle) lamps do use them.

Yes, this should work, but as others have said, it will kill the power
factor.  Also, this will require a HUGE inductor, something like 30+
Kilos of Iron!  Someone also commented about getting a rectifier.  If you
run the lamp on DC, then the inductor must be before the rectifier.  On
DC, the + and - carbons need to be a different size.  The + carbon
receives electrons, and so runs hotter.  You use a larger carbon there
to absorb and dissipate the heat better.  Note that the crater in the
+ carbon is cradling a 20,000+ K plasma, and the UV is catastrophically
dangerous.  One quick peek without adequate protection, and your retina
is fried!

: - Why do iron electrodes not work (it only sparks slightly) ?

You should be able to make an iron/carbon arc lamp work.  You could
probably make an iron/iron lamp work, too, if you could get it started.
A welder does this, by the way.  The trick is to have a huge inductor
built into the welder unit, to provide a massive stabilizing effect on
arc current.  The Carbon is in powdered form, and is a poor heat conductor
in that form.  Therefore, it boils off a bit when the ends touch, and
easily starts the plasma.  If you could make the iron electrodes have a
similar low thermal conductivity, you could make an iron arc lamp start
as easily as a carbon arc.

---

From: [email-masked] (Mick Collins)
Date: Tue, 11 Jun 1996 09:17:36 BST

> - Why do iron electrodes not work (it only sparks slightly) ?
 
  I'll have a stab at that one :-  (all figs VERY approx)

          1500w @ 220v = 1500/220 = 6.8amps 
 
           6.8a @ 220v = 220/6.8 = 32ohms (resistance of ballast)

 BEMF across an arc = 40v, leaving 220 - 40 = 180v across ballast

           180/32 = 5.6amps (current through arc)

           40 x 5.6 = 225watts (power or heat in arc)            

 Carbon is a poor conductor of heat and this wattage concentrated
in the small volume of the arc raises the temperature enough to
keep the carbon vapourised.
 Iron is a comparatively good conductor and heat flows away too
fast to allow the temperature to get anywhere near high enough to
vapourise iron.  It needs far more than 225 watts to maintain an
arc with iron - note typical ratings of arc welders and multiply 
by 40 to get some idea of how many.
  -- 
     Cheers

  Mick  S.M.E.E.           "From the inventors shop came
                                 the sound of hammering tin"

---

From: [email-masked] (Robert Galloway)
Date: 11 Jun 1996 06:29:33 -0600

In article <[email-masked]>, C++ Freak  <[email-masked]> wrote:
>arc would take place. 
>a nail in the arc disrupted it. 

The voltage drop accross an arc is proportional to it's length.  
Additional applied voltage will increase the current flow without much 
increase in voltage drop accross the arc.  The principle is the same as 
that in the old gas voltage regulator tubes.

Then I had to switch off power, 
>over each other used normally for an oxyacetelene torch.
I'd recommend against that.  Get a lens specifically for viewing the 
electric arc.  A cheap arc weldor's helmet would be the simple solution.

>- How can I feed metal into the arc without disrupting the arc so that it melts ?

I wouldn't try to feed it into the arc.  Depending on what quantity of 
metal you want to melt, you could angle the carbons so the arc is 
directed towards the metal as a flame or strike the arc between the 
carbons and the metal to be melted.  A carbon to carbon arc is easier to 
strike and maintain but an arc between the metal and a carbon will work 
and is commonly used.

>  like an electric heater ? 

An inductive ballast will work very well.

>- Why do iron electrodes not work (it only sparks slightly) ?

Don't know.  I suspect it has to do with the amount and conductivity of 
vapor put off by the metal versus that from carbon.

>Klaas
I tried the same thing in 1958 with two non-coated welding carbons, four 
500W light bulbs in parallel on a piece of concrete.  The superintendant 
of schools and principal saw it and thought it was neat and asked me to 
demo it up in the new science lab.  I was too lazy to carry the piece of 
concrete so we dug up a bunch of Transite (asbestos and portland cement) 
board and built up a stack about 3/4".  The demo was melting and boiling 
glass.  After about half a minute, a burning smell tipped me off we were 
in trouble.  The molten glass had gone right through the transite and 
through the surface of the lab bench.  And *I* caught all the hell. 8-)

rhg

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From: Jeffrey Prothero <[email-masked]>
Date: Tue, 11 Jun 1996 16:41:52 -0700

This is not something that I have tried, but I have seen references to
using a magnet to blow the arc into a crusible....As I remember it was in
"Procedures in experimental Physics" available as a reprint from Linsay.

---

From: [email-masked] (Nick Irias)
Date: Wed, 12 Jun 1996 00:01:04 GMT

>- How can I feed metal into the arc without disrupting the arc so that it melts ?

Use the copper clad carbon rods at about a 60 degree angle to one
another.  They will project an arc past the intersection point of
their centerlines.  It looks almost like a torch flame.  I dont know
exactly why this works, I've just used an adjustable carbon arc torch
before and played with the angles.

>  like an electric heater ? Gas-discharge (based on the same principle) 

The most efficient power supply, if you want to use the carbon arc for
heating or torching, is to power it with a 100% duty cycle welder.
I've only done this with a cheap AC welder.  A DC welder would
probably work also.

---

From: C++ Freak <[email-masked]>
Date: 12 Jun 1996 06:57:11 GMT

>"Procedures in experimental Physics" available as a reprint from Linsay.
I will try to and moreover use an electric welder up to 150 amps.
The latter for safety (you don't have to worry about touching live parts) and
the availability of an inductive ballast. 
I don't know whether a welder provides DC or AC .
I want to attach a graphite crucible on one electrode and a carbon rod as the
other which arcs on the metal to be melted.
Is this a better approach ?

Klaas

---

From: [email-masked] (Dave)
Date: Tue, 11 Jun 1996 23:29:17 GMT

[email-masked] (Jonathan M. Elson) wrote:
[snip]
> If you

 At least one World War II searchlight used a DC carbon arc. The
carbons were the same diameter, but the negative was solid carbon,
while the positive had an iron-powder core.

--  Dave Brooks <http://www.iinet.net.au/~daveb>
PGP public key: finger  [email-masked]
                servers [email-masked]
    fingerprint 20 8F 95 22 96 D6 1C 0B  3D 4D C3 D4 50 A1 C4 34

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From: [email-masked] (COUGER GORDON)
Date: 12 Jun 1996 13:28:33 GMT

>>"Procedures in experimental Physics" available as a reprint from Linsay.

I recall drawings of arc furnaces that only used one carbon electrode and the
melt material for the other. The electode came in from the top and the arc was
formed by plunging it into the metal and with drawing it slightly.

A good welder would be a good source of power.

Gordon

Gordon Couger Senior Software Specialist
Biosystems & Agricultural Engineering Dept.  Oklahoma State Univ.
114 Ag Hall Stillwater, OK 74075
[email-masked]

---

From: Dan Mills <[email-masked]>
Date: Wed, 12 Jun 96 19:59:21 GMT

>while the positive had an iron-powder core.

All carbon arc lamphouses for cine projectors used DC as the flicker would 
have mixed with the second harmonic of the 48hz shutter frequency to give
100 - 96hz = 4hz. This would have been quite visible.
The DC was (and sometimes still is) produced by a saturatable reactor 
driving a copper oxide rectifier and a huge bank of capacitors.
The reason for the different diamerers of the electrodes is that a pit
forms in the positive one and it is from here (not from the arc) that most
of the light is emmitted.
-- 
********************************************************
* And on the first day the lord said...... *           *
*......Lx1, Go! and there was light!       * Dan Mills *
********************************************************
DMX-512 FAQ : ftp.webcom.com/pub1/lightsrc/ftp/dmx512fq.txt

---

From: [email-masked] (Robert Galloway)
Date: 13 Jun 1996 12:25:31 -0600

>while the positive had an iron-powder core.
I think you'll find that the main advantage to two sizes of carbons for a 
DC carbon arc is where a clockwork feed is used.  The positive carbon is 
consumed faster than the negative.  If they are sized right you can feed 
the two different sized carbons at the same rate and they stay even.

rhg

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From: [email-masked] (Ian Grey)
Date: Fri, 14 Jun 1996 00:17:20 GMT

>the two different sized carbons at the same rate and they stay even.

I remember my local Cinema from 25 years ago, it had Italian Victoria 8s
running at 120 Amps (It was a big house!)They ran 6000ft spools (60
minutes), the positive carbon was about 2'6" long with the crater
focussed onto a large dichroic mirror. The negative was about 1' long
passing through a central hole in the mirror. Although the negative
diameter was slightly smaller than the positive, the negative lasted 3
spools whilst the positive had to be replaced every change-over. they
were cored and copper coated and powered via limiting resistors from a 4
bottle mercury arc rectifier suite. They were motorised but had to be
trimmed by hand if it was windy!.

They also had two Stelmar spots, they used non-coated carbons with hand
gearing. I can't remember much about changing rods as I only operated
them a few times but the positive was about 1" diameter and we couldn't
get much more than an hour and a quarter out of them which meant losing
one then the other for a long show. (It wasn't realistic to douse them
between numbers as striking them up burnt off as much as we had saved).
 _____________________________________________________________________
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From: Willis Eschenbach <[email-masked]>
Date: Thu, 13 Jun 1996 17:20:33 -0700

As a kid, I worked for an old blueprint shop. An enormous ancient 
blueprint machine used carbon arcs to make the prints.

I was fascinated by the machine. When I traced out the regulation of the 
feeding of the rods, I was amazed by its simplicity. 

The power to the arc first ran through a mondo sized solenoid. One rod 
was attached to the moving part of the solenoid. A spring kept the rods 
touching when the machine was off.

When the switch was turned on, the solenoid received full current, so it 
pulled the rods apart (against the spring). The further it pulled, the 
less current ran through it, so it stopped before the arc was 
extinguished. At that point it was perfectly balanced. It would even 
restart automatically if for some reason it lost the arc ...

I thought ... wow ...

willis

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From: [email-masked] (Robert Galloway)
Date: 13 Jun 1996 12:30:40 -0600

>I don't know whether a welder provides DC or AC .

They come both ways.  Transformer AC - DC welders are fairly common 
anymore.  Why didn't you tell us you had access to a welder? 8-)

rhg

---

From: C++ Freak <[email-masked]>
Date: 17 Jun 1996 09:44:07 GMT

>anymore.  Why didn't you tell us you had access to a welder? 8-)

I tried my newly bought welder and put it to 50 Amps (max possible 150 A)
and it worked much better.
But keeping the arc sustained is still a problem and putting a rectifier
between
it makes things even worse.
Using a crucible (1 " inner diameter) works too, but heating occurred only 
locally, but, anyway, it melts a few grams of iron in seconds. But it
leaves pits in the crucible wall on the location of the arc.
Using a piece of charcoal with a carbon battery rod heats quicker.

Klaas