Film-Tech Forum ARCHIVE: Striking an arc

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Posted by Jim Spohn (Member # 1547) on 05-15-2003, 09:48 AM:

The method of striking an arc with a zenon bulb is of course
a high voltage spike across the electrodes to ionize the space between them and therefore create a path of conduction. Does anyone know if that same technique would work with striking a arc in a carbon-arc lamp, assuming that the correct trim distance, (or closer)is between the carbons, then let the feed motors adjust the trim ?....Jim

Posted by Gordon McLeod (Member # 33) on 05-15-2003, 10:11 AM:

It will work but the odds are against the arc staying lit as it will probably blow the core out of the postive carbon when it lights

Posted by John Pytlak (Member # 331) on 05-15-2003, 10:44 AM:

I remember trying little ignitor links that were sold in the late 1960's to try to automate lighting a carbon arc lamp. The link was placed between the negative and positive carbons, and when the lamp was energized, the link would vaporize and SOMETIMES light the arc. [Roll Eyes]

Does anyone remember what these were called, and who sold them?

Posted by Stephen Furley (Member # 1277) on 05-15-2003, 11:38 AM:

There was a type of arc lamp known as a Jablochkoff Candle, which had two parallel carbon rods a few millimetres apart, with the gap between them filled with an insulating material {magnesia?) The tops of the rods were joined by a small carbon bridging piece, and the burning away of this, when the current was applied struck the arc. The insulating material just crumbled away as the rods burned down. Sometimes several 'candles' were mounted together, each one striking automatically when the previous one burned out. They had to be operated on a.c. to ensure an equal rate of consumption of the two electrodes. The idea was to avoid the complexity of an automatic feed system, but I've seen one operating, and they didn't give a very good light. Also, if the arc went out for any reason, it would not re-srrike.

They were used briefly for street lighting, I believe that the Victoria Embankment, in London, was lit by them for a while.

Posted by Paul G. Thompson (Member # 655) on 05-15-2003, 01:25 PM:

John, I used them, too. But I also forgot what they are called.

I made my own. Real simple. Just take a used carbon rod, smash it with a hammer and use a small fragment that contains part of the core.

They had (the ones I made) had about a 66% hit, but they were not worth the hassle.

Posted by Jim Spohn (Member # 1547) on 05-15-2003, 02:18 PM:

I knew that the very clever guys on Film Tech would have the answers!! Thank you gentelmen......You have given me many ideas to play with......Jim

Posted by Pete Naples (Member # 761) on 05-15-2003, 02:46 PM:

Those pellets were still around in the 80's and early 90's. Evil things, they showered the innards of the lamphouse with crud when they vaporised. If you didn't have automatic rear dowsers (a la Magnarc), or you hand't closed the mirror dowser the life of the shiny bowl shaped part became somewhat reduced.

I seem to recall them being marketed for use with Cinemation, and something to do with the Fedi 'monster' is ringing a bell in my grey matter too.

Posted by Bernard Tonks (Member # 809) on 05-15-2003, 03:05 PM:

They were called copper slugs, supplied to cinemas with Projectomatic automation with the drum. Later models were Cinemation, the first types with a drum, then matrix boards. I worked at one cinema that had Projectomatic ‘0’ model which only did changeovers. I found the copper slugs very reliable.

The BTH SUPA carbon arcs struck automatically when switching on manual or automatic, the negative carbon kissed the positive carbon with the aid of a solenoid.

Posted by Paul G. Thompson (Member # 655) on 05-15-2003, 03:55 PM:

Bernard said:

quote:
...the negative carbon kissed the positive carbon with the aid of a solenoid.

That was a great idea. Many moons ago a thought like that went through my mind but I never tried it because I was not that lazy to get off my butt and strike tha arc like a normal projectionist. [Big Grin]

Posted by Tim Reed (Member # 137) on 05-15-2003, 04:20 PM:

quote:
Does anyone remember what these were called, and who sold them?

I have a box of these somewhere. Mine are Nationals.

Posted by Jim Spohn (Member # 1547) on 05-15-2003, 04:58 PM:

Paul...The reason that I want to strike the arcs automaticly is that every thing in my theatre is remote control from both stationary and hand-held remotes, house-lights, curtains, stage lights and booth etc. There are times when I have to play the pipe organ myself then run to the booth and start the movie (10 minute silent) then run back down and play for the movie. When the film is over I can shut down the projector via remote. It seems like a good idea to be able to start the film from remote also. There is nothing here to say that I'm not a lazy-ass because I am, but I just don't want to be a track star!! I know that a projectionst is the real answer, but that is'nt always feasable....Jim

Posted by Paul G. Thompson (Member # 655) on 05-15-2003, 05:02 PM:

John, sadly I must say that it is probably time to retire the arc lamps since you are in that predicament [Frown]

Posted by Robert Throop (Member # 61) on 05-15-2003, 07:11 PM:

The Peerless Hi Lo made in the late twenties also had a solenoid operated striker.

Posted by Paul G. Thompson (Member # 655) on 05-15-2003, 08:26 PM:

If one was to home-brew a modification of a carbon arc lamp to use a solenoid for arc striking, the Simplex High and the Enarc would be the easiest to do, as they have striking triggers that are not ganged with the feed motors either directly or indirectly.

The rest that I know of requires the operator to rotate a crank or a knob (as an example, Ashcan's, Peerless, Strong, RCA Hi-Arc, Wide Arc, etc.) to kiss the carbons.

Robert, isn't that an old "Ashcan" you have there that uses the "Black Stick?" I am confused, because it looks like an RCA emblem on it. I could be wrong, but didn't Ashcraft build some for RCA?

Posted by Robert Throop (Member # 61) on 05-15-2003, 10:19 PM:

Paul,
That's a Brenkert Supertensity condenser arc. It burned 13.6mm carbons at currents up to 180 amps. It was Brenert's answer to the Peerless HyCandescent.

Posted by Jack Ondracek (Member # 1466) on 05-15-2003, 11:10 PM:

Unless you had a lamp (like the one Strong made) that used some form of automatic alignment mechanism, how could any automatic form of striker give you a consistent trim?

Posted by John Stewart (Member # 989) on 05-15-2003, 11:20 PM:

For a lack of better terms, I think we called them strikers. Ours were not made of copper but were the same material as the rods themselves. Carbon maybe. The ones we used were pointed on one end and the other had a "v" shape that allowed you to position that end on the edge of the crater. I positioned them at the same angle that the carbon rotated and they were kicked out as they struck. Didn't have any problems with them other than small fragments on the bottom of the housing. They were fairly reliable as I recall. Of course, I'd rather be on hand for change overs but it wasn't always possible. We were making automated change overs on 5k reels in that booth and 6k in the other with a lobby between the two. It was the Mann's Fox in Austin. We were running Norelco AA in that house with Strong lamps. The other house was Xenon.

John

Posted by Rick Long (Member # 213) on 05-15-2003, 11:24 PM:

Back in the 70's Barry Blackburn and I (both working for General Sound and Theatre Equipment in those days) worked out a way for a Kneisley ignitor to strike a carbon 7 & 8 trim in a Peerless Magnarc. Worked every time.
If failing memory serves, it seems we had to use a relay to disconnect the arc-feed motor while striking though.

Posted by James Robertson (Member # 888) on 05-16-2003, 05:39 AM:

Bernard "They were called copper slugs, supplied to cinemas with Projectomatic automation with the drum. Later models were Cinemation, the first types with a drum, then matrix boards. I worked at one cinema that had Projectomatic ‘0’ model which only did changeovers. I found the copper slugs very reliable."

I worked the original Projectomatic in the late 50's using the slugs to strike Peerless Magnarcs.
The problem was that the automation struck the arc on the motor cue which only gave 7secs for the arc to stabilise before changeover. Not even a Magnarc is that good so often the light was unsatisfactory on changeover.

We solved the problem by striking manually about a minute before changeover which sort of defeats the purpose of automation!

Jim

James

Posted by Jim Spohn (Member # 1547) on 05-16-2003, 09:15 AM:

Paul... You're right about Ashcraft making lamps for RCA. My lamps have an RCA emblem on them but were clearly made by Ashcraft. RCA called these lamps "BRITE ARC" they are 120 amp high intensity, water cooled and rotating carbon. The unit that rotates the carbon is a ratchet affair and makes a click - click - click sound that takes a while to get used to. Give up my carbon arc lamps??? NEVER!! (I do have a couple of Super Lumex just in case I can't get carbons any longer.)....Jim

Posted by Tim Reed (Member # 137) on 05-16-2003, 10:51 AM:

Wide-Arc and Dyn-Arc were RCA "versions" of Ashcraft Cinex, and Super Cinex, respectively. There were other models, too. The Brite-Arc may be an Ashcraft Hydro-Arc.

Posted by Gordon McLeod (Member # 33) on 05-16-2003, 02:31 PM:

Union Carbide made the pellets to autostrike
Raven Labd made a rotary solidoid tht mounted onto the back of the slip clutch for the negative feed of the magnarc with a current sensing relay to auto strike them. It worked every time
Morganite also built the auto arc that had a solinoid striker in it

Posted by Paul G. Thompson (Member # 655) on 05-16-2003, 03:18 PM:

Jim, I am surprised you can find carbons for that thing. I had some 11mm black stick lamps I ran for years, and near the closing of that theatre (Drive-in) we found that carbons were getting extremely expensive...

My lamps were (if I recall corectly-been over 25 years now) were called the Hi-Arc. It didn't have the ratchet assy to rotate the positive carbon. It was just a chain drive to the collett.

Posted by Robert Throop (Member # 61) on 05-16-2003, 03:21 PM:

The RCA Brite Arc sounds like the Ashcraft Super High. I don't believe the Hydro Arc rotated the positive carbon.

Posted by Paul G. Thompson (Member # 655) on 05-16-2003, 03:31 PM:

If the neg carbon was off-set, the positive carbon had to rotate. Without rotating, one helluva toenail would develope on the positive carbon. If it were a head-on feed, rotation of the positive carbon was not reqired unless there was a reason I don't know about.

Posted by Robert Throop (Member # 61) on 05-17-2003, 01:17 PM:

The carbons in a Hydro Arc were coaxial like in a suprex lamp.

Posted by Frank Angel (Member # 248) on 05-17-2003, 06:06 PM:

Paul mentions that a "toenail" would develope on the negative carbon if it didn't rotate, but on occasion I have seen such a lip develop on the neg carbon in a Magnarc lamphouse (non-rotating carbons). Anyone know exactly why this happens? I haven't been able to pinpoint what conditions cause it, but when it does happen, it can drop light output to almost nothing and cause the drive motor to go wild. There is a tell-tail hissing sound the arc makes when this gets going. If you don't hear it and are not paying attention, your screen can go nearly black.

You can get rid of the lip by smashing the two rods together for a moment and that will break it off, but that solution causes a momentary near-blackout on the screen. I found that using a thick screwdriver and reaching into the lamphouse and giving the tip a good whack with it will break it off and not be quite as disruptive to the screen image. It also gives you a nice sunburn on your forearm. [Eek!]

Frank

Posted by Gordon McLeod (Member # 33) on 05-17-2003, 06:23 PM:

Two common problems To short an arc causes mushrooming and too high of current will cause spindling where the neg gains a long slender point
Also there can be a problem with formation of a carbide tip on the negative that will make it hard to strike

Posted by Paul G. Thompson (Member # 655) on 05-18-2003, 12:57 AM:

I have seen spindling as Gordon has described, mut never a toenail on a negative rod. However, I have seen a "ball" form on the tip that drives the arc nuts. Getting rid of it was easy to do if you smash the negative carbon into the positive.

Of course, a couple of times I got a little too hefty the negative feed knob....and broke the positive carbon off slicker than snot on a brass doorknob. [Eek!]