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Progress report on mid sized VDG

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  • jpiperson2002
    Things are progressing a bit slower than expected but are finally coming together. I d originally planned to use a small 12-24V motor with a 50W rheostat to
    Message 1 of 8 , Aug 7, 2007
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      Things are progressing a bit slower than expected but are finally
      coming together. I'd originally planned to use a small 12-24V motor
      with a 50W rheostat to adjust the speed, it worked fine without the
      belt but didn't have enough power to spin the belt once it was added.
      So I switched to small variable speed AC motor with the electronics
      sheltered within a grounded foil lined box. Hopefully this will
      protect the controller from shorting out once high voltages are being
      produced, if not I'll switch to a larger single speed AC motor.

      I ended up using clear (actually somewhat translucent with a blueish
      tint) schedule 40 PVC for the column. It's cut to about 38 inches
      between rollers, it was 42" originally but the Theraband rubber belt
      I'm using was stretched too tight at that length. It was too expensive
      for the clear PVC for the hazy optical clarity and if I did it over
      again I'd probably use clear acrylic tubing instead. The commonly
      available clear acrylic tubing only has a wall thickness of 1/8 inch
      though, compared to 1/4 wall for the schedule 40 PVC, and it's easier
      to mount the roller bearings in the thicker wall of the PVC.

      I found a discarded single drawer wood night stand to use for the
      base, removed the legs and lined the inside with grounded metal foil
      to act as a Faraday cage for the motor and controller. Then used a
      couple of large plastic cutting boards insulating sheets, mounted one
      under the wood base and another on top of the base to hold the
      grounding probes.

      I'm using an inverted 14" Ikea Banda blank stainless steel bowl for
      the base of the column and a temporarily using a couple of 14" Banda
      b;lank bowls for the top terminal. I've had a 16" gazing ball on
      backorder for the top terminal for a couple of months already and will
      ultimately use that on the completed device. I'd actually received a
      20" gazing ball but after calculating the capacitance at around 8
      Joules decided to return it as too powerful. Oddly enough there don't
      seem to be any 18" stainless steel gazing balls available online, just
      16" then 20". Shop around for best deals on gazing balls my 16" ball
      cost about $75US including shipping.

      There's only one easy way to cut an access hole in a stainless steel
      gazing ball, have someone do it for you using a plasma torch, Despite
      the impressive name a plasma torch is just a small (the size of a
      toaster) electric torch which strikes a low voltage spark and blows
      air across the spark, you don't even need welders goggles for the
      glare (though safety glasses are always a must). The job will be made
      much easier and neater if you cut a template for the hole in a piece
      of 1/4 plywood and then trace the torch along the inside of the hole.
      It takes less than a minute to cut an accurate 4" hole with rough
      edges using a template, and then another 5 minutes with a grinding
      stone in a power drill to smooth it off. A local car mechanic had a
      plasma torch and did the job for free, there is no other really easy
      way to cut a hole in stainless steel.

      Since my last post my sons and I have made a nice Jacob's ladder
      (that's where the clear acrylic tubing I'd intended for the VDG ended
      up) and we also picked up a beautiful solid state tesla coil on eBay
      http://www.teslacoil.net/3.8/3.8fpay.htm
      Both of those devices are very dangerous compared to a VDG and we've
      been having lots of safety talks about the difference.

      I've also done some capacitance tests of different size plastic
      bottles in salt water, and of foil lined 3.5 gal and 5 gallon plastic
      buckets. That was interesting, common 1/2 gallon soda bottles had the
      highest capacitance by far averaging about 7.5nF per bottle, by
      comparison a 3.5 gallon bucket lined with aluminum foil only came in
      at about 3.5nF. Since capacitance grows as the square of the voltage,
      the lower capacitance bucket may still store a considerable amount of
      charge since it can be charged to several tens of thousands of volts
      before it shorts out through the plastic.

      Four 1/2 gallon soda bottles full of salt water in a 5 gallon bucket
      of salt water would hold about 30nF and would store about 1.5 Joules
      at 10Kv or 3.4 Joules at 15kV (probably the max voltage for thin
      plastic like this). This is about the same amount of charge which is
      held in a VDG 14" top sphere, though the current in the capacitor
      would be larger and more considerably more dangerous since the 10KV
      voltage in the capacitor since the capacitor voltage is almost 50
      times lower than the voltage on a VDG sphere.

      By way of comparison, it's easy to find a 400V electrolytic pulse
      capacitor a few inches long for about $10 which is capable of storing
      over 35 Joules. So Leyden jars and salt water capacitors are
      interesting as historical artifacts, and they can be moderately
      dangerous if used in a sparking demo with a Van de Graaff, but they
      are very weak by comparison to modern capacitors.

      John Piper
    • Richard Linder
      Hi John: Since rehostats are basically a variable resistance in series with the load and source voltage, the result is a poorly regulated power source for the
      Message 2 of 8 , Aug 10, 2007
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        Hi John:

         

        Since rehostats are basically a variable resistance in series with the load and source voltage, the result is a poorly regulated power source for the motor. The motor is a dynamic load which is dependent on the mechanical load on the motor shaft and the applied voltage. This results in poor speed regulation and unpredictable starting characteristics.  I also tried SCR control, the electronics being located in a metal box at the end of a 15 foot shielded cable. No way!  The first arc wiped it out. Terrifying transients running down the control cable.  The latest and successful system is a universal motor powered by a powerstat (Variac).

         

        A source for acrylic tubing is Lifereef Systems. They are on the web. Acrylic is very costly and I did not use it on my 2nd large VDG. Instead I used an open column design using flat fiberglass strips. Take a look at my pictures. The small VDG uses a 18” gazing ball and the large one uses a 30” spun aluminum hemisphere which was welded to a matching toroid.

         

        Good for you on the plasma torch!!!  On the 18” sphere I did it the hard way, with a nibbling tool. The edge of the hole was so rough that I had to make a copper tubing corona ring and silver solder it to the opening in order to shield the rough edges.

         

        Your experiments with plastic bottles are very interesting. A science teacher friend of  mine built a Tesla coil using brine filled / foil covered beer bottles. Needless to say he has to keep several spares on hand when we tour the local schools during the annual electricity show.  As part of the VDG demo I charge a five gallon plastic Leyden “bucket” with the VDG. I place a  grounded 2” polished sphere about 4 inches from the HV sphere of the bucket.  The VDG arcs to the bucket terminal and when the voltage reaches the breakdown point, there is a brilliant arc to the grounded sphere accompanied by a very loud bang. I have not calculated the energy of the bucket but it is deadly.

         

        Best wishes,

        Dick Linder

          

         

      • john_d_farr
        John and Dick: I m building a medium sized machine. I ve not found any reasonably priced spheres yet. I ve looked at gazing balls, but the best price I ve
        Message 3 of 8 , Aug 11, 2007
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          John and Dick:

          I'm building a medium sized machine. I've not found any reasonably
          priced spheres yet. I've looked at gazing balls, but the best price
          I've found is nearly $50 with shipping. I'd like to go larger. Any ideas?

          Thanks, John

          --- In VanDeGraaffGenerator@yahoogroups.com, "Richard Linder"
          <linder@...> wrote:
          >
          > Hi John:
          >
          >
          >
          > Since rehostats are basically a variable resistance in series with
          the load
          > and source voltage, the result is a poorly regulated power source
          for the
          > motor. The motor is a dynamic load which is dependent on the
          mechanical load
          > on the motor shaft and the applied voltage. This results in poor speed
          > regulation and unpredictable starting characteristics. I also tried SCR
          > control, the electronics being located in a metal box at the end of a 15
          > foot shielded cable. No way! The first arc wiped it out. Terrifying
          > transients running down the control cable. The latest and
          successful system
          > is a universal motor powered by a powerstat (Variac).
          >
          >
          >
          > A source for acrylic tubing is Lifereef Systems. They are on the web.
          > Acrylic is very costly and I did not use it on my 2nd large VDG.
          Instead I
          > used an open column design using flat fiberglass strips. Take a look
          at my
          > pictures. The small VDG uses a 18" gazing ball and the large one
          uses a 30"
          > spun aluminum hemisphere which was welded to a matching toroid.
          >
          >
          >
          > Good for you on the plasma torch!!! On the 18" sphere I did it the hard
          > way, with a nibbling tool. The edge of the hole was so rough that I
          had to
          > make a copper tubing corona ring and silver solder it to the opening in
          > order to shield the rough edges.
          >
          >
          >
          > Your experiments with plastic bottles are very interesting. A science
          > teacher friend of mine built a Tesla coil using brine filled / foil
          covered
          > beer bottles. Needless to say he has to keep several spares on hand
          when we
          > tour the local schools during the annual electricity show. As part
          of the
          > VDG demo I charge a five gallon plastic Leyden "bucket" with the VDG. I
          > place a grounded 2" polished sphere about 4 inches from the HV
          sphere of
          > the bucket. The VDG arcs to the bucket terminal and when the voltage
          > reaches the breakdown point, there is a brilliant arc to the
          grounded sphere
          > accompanied by a very loud bang. I have not calculated the energy of the
          > bucket but it is deadly.
          >
          >
          >
          > Best wishes,
          >
          > Dick Linder
          >
        • Richard Linder
          John: My small VDG uses an 18 stainless gazing ball and if I recall correctly I paid around $75 plus shipping from a garden supply place in the south eastern
          Message 4 of 8 , Aug 12, 2007
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            John:

             

            My small VDG uses an 18” stainless gazing ball and if I recall correctly I paid around $75 plus shipping from a garden supply place in the south eastern part of the U.S. (I live near Boston ). This was 5 years ago so I imagine costs are +++ these days. The Big VDG uses a 36” aluminum sphere made up of a hemisphere welded to a toroid. This beauty cost $350 but fortunately it was manufactured in Western Massachusetts and could be picked up rather than shipped.  Even more fortunately, I was subsidized by the local school department since I was going to be an adjunct to the school science department’s annual static electricity show. I searched the web for days on end and I am afraid there are no inexpensive spheres out there. I think the closest substitute would be stainless steel or aluminum large salad bowls which are used by food service companies. But, you have to deal with the seam where the two join. Possibly a skilled welder could join them and grind the weld smooth. The joint between my toroid and hemisphere is invisible!  So, a skilled welder may do the trick.

            Dick     


            From: VanDeGraaffGenerator@yahoogroups.com [mailto: VanDeGraaffGenerator@yahoogroups.com ] On Behalf Of john_d_farr
            Sent: Saturday, August 11, 2007 11:10 PM
            To: VanDeGraaffGenerator@yahoogroups.com
            Subject: [VanDeGraaffGenerator] Re: Progress report on mid sized VDG

             

            John and Dick:

            I'm building a medium sized machine. I've not found any reasonably
            priced spheres yet. I've looked at gazing balls, but the best price
            I've found is nearly $50 with shipping. I'd like to go larger. Any ideas?

            Thanks, John

            --- In VanDeGraaffGenerato r@yahoogroups. com, "Richard Linder"
            <linder@...> wrote:

            >
            > Hi John:
            >
            >
            >
            > Since rehostats are basically a variable resistance in series with
            the load
            > and source voltage, the result is a poorly regulated power source
            for the
            > motor. The motor is a dynamic load which is dependent on the
            mechanical load
            > on the motor shaft and the applied voltage. This results in poor speed
            > regulation and unpredictable starting characteristics. I also tried SCR
            > control, the electronics being located in a metal box at the end of a 15
            > foot shielded cable. No way! The first arc wiped it out. Terrifying
            > transients running down the control cable. The latest and
            successful system
            > is a universal motor powered by a powerstat (Variac).
            >
            >
            >
            > A source for acrylic tubing is Lifereef Systems. They are on the web.
            > Acrylic is very costly and I did not use it on my 2nd large VDG.
            Instead I
            > used an open column design using flat fiberglass strips. Take a look
            at my
            > pictures. The small VDG uses a 18" gazing ball and the large one
            uses a 30"
            > spun aluminum hemisphere which was welded to a matching toroid.
            >
            >
            >
            > Good for you on the plasma torch!!! On the 18" sphere I did it the
            hard
            > way, with a nibbling tool. The edge of the hole was so rough that I
            had to
            > make a copper tubing corona ring and silver solder it to the opening in
            > order to shield the rough edges.
            >
            >
            >
            > Your experiments with plastic bottles are very interesting. A science
            > teacher friend of mine built a Tesla coil using brine filled / foil
            covered
            > beer bottles. Needless to say he has to keep several spares on hand
            when we
            > tour the local schools during the annual electricity show. As part
            of the
            > VDG demo I charge a five gallon plastic Leyden
            "bucket" with the VDG. I
            > place a grounded 2" polished sphere about 4 inches from the HV
            sphere of
            > the bucket. The VDG arcs to the bucket terminal and when the voltage
            > reaches the breakdown point, there is a brilliant arc to the
            grounded sphere
            > accompanied by a very loud bang. I have not calculated the energy of the
            > bucket but it is deadly.
            >
            >
            >
            > Best wishes,
            >
            > Dick Linder
            >

          • jpiperson2002
            ... ... I m building a medium sized machine. I ve not found any reasonably priced spheres yet. I ve looked at gazing balls, but the best price I ve found
            Message 5 of 8 , Aug 12, 2007
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              --- In VanDeGraaffGenerator@yahoogroups.com, john_d_farr wrote;
              ... > I'm building a medium sized machine. I've not found any
              reasonably > priced spheres yet. I've looked at gazing balls, but the
              best price > I've found is nearly $50 with shipping. I'd like to go
              larger. Any ideas?
              >
              > Thanks, John

              The least expensive place online which I've found recently for
              stainless steel gazing balls is;
              http://www.exteriorguide.com/Accessories.html
              12" $29, 16" $59 and 20" $79 plus about $15 for shipping, the 20" in
              particular is a good deal at less than 1/2 the cost of other 20" balls
              I've seen recently. Note that my 16" ball just received from this
              vendor was actually measured about 15 1/4 inch diameter, it arrived 5
              days after the order was placed. Even if the sizes run a bit smaller
              than advertised, a 19"-20" sphere would still be a formidable unit
              storing about 8 joules, compared to about 4 Joules for a 16inch
              sphere. Surprisingly I wasn't able to find any 18" stainless steel
              gazing balls, I would have preferred 18" over 16" while 20" is just
              too large for me.

              If I recall correctly the big Science First 17" replacement aluminum
              sphere for their Van de Graaff generators costs about $225 but it
              comes with a nicely shaped entry point for the column. That would be
              more efficient in terms of reduced loss to corona discharge compared
              to the edges of a sharp entry hole cut into a gazing ball, but ti
              wouldn't nearly be as much fun...
            • jpiperson2002
              I hope that I m not breaking VDG forum etiquette by adding these occasional progress updates as replies to my own thread. My VDG project has turned out to be
              Message 6 of 8 , Sep 19 9:10 PM
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                I hope that I'm not breaking VDG forum etiquette by adding these
                occasional progress updates as replies to my own thread. My VDG
                project has turned out to be more involved than expected and you may
                be interested in the way it's evolving.

                Since the last post I received an 18.5 inch gazing ball, cut a 6 inch
                access hole with a plasma torch and smoothed the edge with a grinding
                stone. Then slit a piece of 1/4 inch clear vinyl tubing along one side
                and packed it with anti-corona putty, then mounted it along the
                semi-rough access hole. Also ordered a 5 foot long piece of 5 inch ID
                white PVC from Cole Palmer and a 1 foot section of 2.5 in OD nylon rod
                for the rollers. (I had been planning to use a piece of 3 inch ID
                clear PVC but decided that the charging current wouldn't be high
                enough with a narrow tube like that.)

                Got it all roughed together but so far have not been able to drill a
                sufficiently centered hole in the heavy Nylon rod for the roller to
                run smoothly at speed. I have a drill press and a centering tool but
                no lathe and just haven't been able to drill a sufficiently centered
                mounting hole for the bearings and the rollers made so far weren't usable.

                So then I cut out a number of 4 inch long 2 inch ID (2.5 inch OD)
                sections from a PVC tube and glued in end plugs cut from 1/2 thick
                plastic cutting board. Those are so much lighter than the Nylon rod
                that the centering of the axle hole isn't as critical, but I still
                haven't managed to get two rollers which run smoothly or quietly at
                speed. The PVC rollers are so inexpensive though that I'll eventually
                find the time to make a bunch of them and chances are that at least 2
                of them will work correctly. I've temporarily put the project to the
                side until the weather gets cold here in Chicago because it was taking
                up too much of my limited free time.

                In the meantime I've found a good deal on a Science First model 10-285
                VDG with a 440 mm (17.3 inch dia) top sphere which should arrive
                within a couple of weeks. It only uses a 3 inch ID column so the
                charging rate will be relatively slow for that size sphere maybe 3 to
                5 seconds, but it's a well reviewed commercial model and is more
                compact than the one I've been making. Plus I know it will work
                correctly from the start, whereas the one I've been working on may
                take months of part time work and tweaking before it's working
                correctly.

                Now I don't really need two relatively large VDGs like this. But since
                that's the way the project is evolving I'll take the opportunity to
                switch polarity on the one I'm making by running a Nylon covered
                roller on the top and Teflon covered roller on the bottom. Dick had
                mentioned that he was surprised and somewhat disappointed by the
                results of his test when arcing a negative VDG to a positive VDG. I'll
                make the same test, maybe my results will be better or maybe worse.
                Either way it will be interesting to compare the specs of the Science
                First commercial machine to my somewhat larger home grown version. If
                I include the many hours of time already spent working on this project
                and the cost of materials, the actual costs of both machines will end
                up being roughly the same.

                It's been a real learning experience though. I've assembled a nice
                little collection of books on static electricity and HV projects plus
                ring binders of info, and in the meantime had a chance to compare the
                capacitance of a few more containers. I'd mentioned previously that 2
                liter soda bottles filled with salt water had a capacitance of 7.5nF
                per bottle. This is considerably higher than the 0.89 nF resulting
                from wrapping a 3.5 gallon food storage quality plastic bucket with
                foil (sides only, didn't cover the bottoms with foil), or the 0.77 nF
                from wrapping a larger 5 gallon bucket purchased from the local
                hardware store. A quick test four 3.5 gallon buckets stacked one
                within the other (about 20 inches high altogether), each with a single
                layer of foil on the outside, had a capacitance of about 2.3nF and
                from what I've read that should be good to over 100KV before flash-over.

                Also made an adjustable spark gap with 1.5 inch threaded balls to
                control the maximum voltage on the capacitor and will eventually use
                the stack as a flashy noise maker to impress the kids. At 80KV (a
                spark gap of about 4.5 inches) the capacitor would only discharge
                about 7 Joules. It'll be potentially dangerous of course but I'm
                extremely safety conscious due to a back ground as a chemist and
                engineer, but the 7 Joules discharged by the bucket capacitor stack
                won't be all that much more than the 5.3 Joules stored in a 17 inch
                VDG sphere.

                So if it's OK with the moderator I'll keep adding posts to this thread
                occasionally as things progress over the next few months. And once I'm
                ready to move on to a different project I'll summarize my results in a
                new thread and post some pictures comparing the design on the two
                machines. That's one of the best things about these topical internet
                forums. Anyone can get advice and post results and have the
                satisfaction of knowing that others of similar interest might find
                something useful or motivating or just plain fun in the exchange.

                John Piper
              • Richard Linder
                Hi John: You said it in a nutshell. the learning experience is terrific and the fellowship on the forum is outstanding. There are some items, cylindrical in
                Message 7 of 8 , Sep 20 1:19 PM
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                  Hi John:

                  You said it in a nutshell… the learning experience is terrific and the fellowship on the forum is outstanding.  There are some items, cylindrical in nature, with pre existing centered holes.  Heavy duty bed casters and commercial bakery rolling pins come to mind. On my big VDG I used sections of rolling pins with a appropriate sleeve pressed onto it. Admittedly it still required some lathe work to achieve a press fit. Another possibility might be plastic wheels as found on toys or similar such items found in catalogs like McMaster-Carr. Wheels can be stacked on a shaft to form a roller.

                   

                  Interesting numbers you have come up with regarding the capacitance of various configurations. I would suggest that even though a 5 gallon bucket seems rather puny as compared to a 2 liter soda bottle, the bucket has to be handled with care because it can charge up to a very high potential.

                   

                  I will be interested in your findings relative two oppositely charged VDGs. 

                   

                  Best wishes, Dick Linder  

                   


                  From: VanDeGraaffGenerator@yahoogroups.com [mailto: VanDeGraaffGenerator@yahoogroups.com ] On Behalf Of jpiperson2002
                  Sent: Thursday, September 20, 2007 12:11 AM
                  To: VanDeGraaffGenerator@yahoogroups.com
                  Subject: [VanDeGraaffGenerator] Re: Progress report on mid sized VDG

                   

                  I hope that I'm not breaking VDG forum etiquette by adding these
                  occasional progress updates as replies to my own thread. My VDG
                  project has turned out to be more involved than expected and you may
                  be interested in the way it's evolving.

                  Since the last post I received an 18.5 inch gazing ball, cut a 6 inch
                  access hole with a plasma torch and smoothed the edge with a grinding
                  stone. Then slit a piece of 1/4 inch clear vinyl tubing along one side
                  and packed it with anti-corona putty, then mounted it along the
                  semi-rough access hole. Also ordered a 5 foot long piece of 5 inch ID
                  white PVC from Cole Palmer and a 1 foot section of 2.5 in OD nylon rod
                  for the rollers. (I had been planning to use a piece of 3 inch ID
                  clear PVC but decided that the charging current wouldn't be high
                  enough with a narrow tube like that.)

                  Got it all roughed together but so far have not been able to drill a
                  sufficiently centered hole in the heavy Nylon rod for the roller to
                  run smoothly at speed. I have a drill press and a centering tool but
                  no lathe and just haven't been able to drill a sufficiently centered
                  mounting hole for the bearings and the rollers made so far weren't usable.

                  So then I cut out a number of 4 inch long 2 inch ID (2.5 inch OD)
                  sections from a PVC tube and glued in end plugs cut from 1/2 thick
                  plastic cutting board. Those are so much lighter than the Nylon rod
                  that the centering of the axle hole isn't as critical, but I still
                  haven't managed to get two rollers which run smoothly or quietly at
                  speed. The PVC rollers are so inexpensive though that I'll eventually
                  find the time to make a bunch of them and chances are that at least 2
                  of them will work correctly. I've temporarily put the project to the
                  side until the weather gets cold here in Chicago because it was taking
                  up too much of my limited free time.

                  In the meantime I've found a good deal on a Science First model 10-285
                  VDG with a 440 mm (17.3 inch dia) top sphere which should arrive
                  within a couple of weeks. It only uses a 3 inch ID column so the
                  charging rate will be relatively slow for that size sphere maybe 3 to
                  5 seconds, but it's a well reviewed commercial model and is more
                  compact than the one I've been making. Plus I know it will work
                  correctly from the start, whereas the one I've been working on may
                  take months of part time work and tweaking before it's working
                  correctly.

                  Now I don't really need two relatively large VDGs like this. But since
                  that's the way the project is evolving I'll take the opportunity to
                  switch polarity on the one I'm making by running a Nylon covered
                  roller on the top and Teflon covered roller on the bottom. Dick had
                  mentioned that he was surprised and somewhat disappointed by the
                  results of his test when arcing a negative VDG to a positive VDG. I'll
                  make the same test, maybe my results will be better or maybe worse.
                  Either way it will be interesting to compare the specs of the Science
                  First commercial machine to my somewhat larger home grown version. If
                  I include the many hours of time already spent working on this project
                  and the cost of materials, the actual costs of both machines will end
                  up being roughly the same.

                  It's been a real learning experience though. I've assembled a nice
                  little collection of books on static electricity and HV projects plus
                  ring binders of info, and in the meantime had a chance to compare the
                  capacitance of a few more containers. I'd mentioned previously that 2
                  liter soda bottles filled with salt water had a capacitance of 7.5nF
                  per bottle. This is considerably higher than the 0.89 nF resulting
                  from wrapping a 3.5 gallon food storage quality plastic bucket with
                  foil (sides only, didn't cover the bottoms with foil), or the 0.77 nF
                  from wrapping a larger 5 gallon bucket purchased from the local
                  hardware store. A quick test four 3.5 gallon buckets stacked one
                  within the other (about 20 inches high altogether), each with a single
                  layer of foil on the outside, had a capacitance of about 2.3nF and
                  from what I've read that should be good to over 100KV before flash-over.

                  Also made an adjustable spark gap with 1.5 inch threaded balls to
                  control the maximum voltage on the capacitor and will eventually use
                  the stack as a flashy noise maker to impress the kids. At 80KV (a
                  spark gap of about 4.5 inches) the capacitor would only discharge
                  about 7 Joules. It'll be potentially dangerous of course but I'm
                  extremely safety conscious due to a back ground as a chemist and
                  engineer, but the 7 Joules discharged by the bucket capacitor stack
                  won't be all that much more than the 5.3 Joules stored in a 17 inch
                  VDG sphere.

                  So if it's OK with the moderator I'll keep adding posts to this thread
                  occasionally as things progress over the next few months. And once I'm
                  ready to move on to a different project I'll summarize my results in a
                  new thread and post some pictures comparing the design on the two
                  machines. That's one of the best things about these topical internet
                  forums. Anyone can get advice and post results and have the
                  satisfaction of knowing that others of similar interest might find
                  something useful or motivating or just plain fun in the exchange.

                  John Piper

                • saihager
                  www.unitednuclear.com lists a 14 globe for their machine at $175. sciencefirst as recent as 11-26-07 lists their 12 globe, #058115 for 129.95, and their 45
                  Message 8 of 8 , Nov 29, 2007
                  • 0 Attachment
                    www.unitednuclear.com lists a 14" globe for their machine at $175.
                    sciencefirst as recent as 11-26-07 lists their 12" globe, #058115 for
                    129.95, and their 45" belt, # 29-1480 for $35.00





                    -
                    -- In VanDeGraaffGenerator@yahoogroups.com, "john_d_farr"
                    <jfarr62@...> wrote:
                    >
                    > John and Dick:
                    >
                    > I'm building a medium sized machine. I've not found any reasonably
                    > priced spheres yet. I've looked at gazing balls, but the best price
                    > I've found is nearly $50 with shipping. I'd like to go larger. Any
                    ideas?
                    >
                    > Thanks, John
                    >
                    > --- In VanDeGraaffGenerator@yahoogroups.com, "Richard Linder"
                    > <linder@> wrote:
                    > >
                    > > Hi John:
                    > >
                    > >
                    > >
                    > > Since rehostats are basically a variable resistance in series with
                    > the load
                    > > and source voltage, the result is a poorly regulated power source
                    > for the
                    > > motor. The motor is a dynamic load which is dependent on the
                    > mechanical load
                    > > on the motor shaft and the applied voltage. This results in poor
                    speed
                    > > regulation and unpredictable starting characteristics. I also
                    tried SCR
                    > > control, the electronics being located in a metal box at the end
                    of a 15
                    > > foot shielded cable. No way! The first arc wiped it out.
                    Terrifying
                    > > transients running down the control cable. The latest and
                    > successful system
                    > > is a universal motor powered by a powerstat (Variac).
                    > >
                    > >
                    > >
                    > > A source for acrylic tubing is Lifereef Systems. They are on the
                    web.
                    > > Acrylic is very costly and I did not use it on my 2nd large VDG.
                    > Instead I
                    > > used an open column design using flat fiberglass strips. Take a
                    look
                    > at my
                    > > pictures. The small VDG uses a 18" gazing ball and the large one
                    > uses a 30"
                    > > spun aluminum hemisphere which was welded to a matching toroid.
                    > >
                    > >
                    > >
                    > > Good for you on the plasma torch!!! On the 18" sphere I did it
                    the hard
                    > > way, with a nibbling tool. The edge of the hole was so rough that
                    I
                    > had to
                    > > make a copper tubing corona ring and silver solder it to the
                    opening in
                    > > order to shield the rough edges.
                    > >
                    > >
                    > >
                    > > Your experiments with plastic bottles are very interesting. A
                    science
                    > > teacher friend of mine built a Tesla coil using brine filled /
                    foil
                    > covered
                    > > beer bottles. Needless to say he has to keep several spares on
                    hand
                    > when we
                    > > tour the local schools during the annual electricity show. As
                    part
                    > of the
                    > > VDG demo I charge a five gallon plastic Leyden "bucket" with the
                    VDG. I
                    > > place a grounded 2" polished sphere about 4 inches from the HV
                    > sphere of
                    > > the bucket. The VDG arcs to the bucket terminal and when the
                    voltage
                    > > reaches the breakdown point, there is a brilliant arc to the
                    > grounded sphere
                    > > accompanied by a very loud bang. I have not calculated the energy
                    of the
                    > > bucket but it is deadly.
                    > >
                    > >
                    > >
                    > > Best wishes,
                    > >
                    > > Dick Linder
                    > >
                    >
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