The Jacobs Ladder project

I've been meaning to try and make one of these for a while and last Sunday seemed to be one of those boring slow days when I wasn't going to achieve anything useful considering the hangover, so I decided to give it a shot.

It's one of those things you used to see in old sci-fi B-Movies where, I guess, they looked hi-tech and exciting. A bolt of buzzing electricity zaps along two metal electrodes that poke upwards in a vee shape. It's pretty impressive when you're up close and personal.

The way it works is that the bottom of the electrodes are close but not touching, so that when a 10,000 Volt HT transformer is hooked up to them the resistance of the air between the shortest points breaks down and a plasma arc is formed across the gap.

The heat of the arc forces it to rise, and because of the electrodes vee shape the arc becomes stretched. If the electrodes are too far apart, then the arc will break, otherwise it will continue to the top of the vee and hold. To force it to break at the top, the ends of the electrodes are bent outwards to stretch the arc so that it breaks every time. A new arc then forms at the base and the cycle begins again.

The dodgy snapshot above was taken from a video clip of my prototype in action. This picture shows the arc at the top of the electrodes and about to snap.

Once I had proved that it would work, I decided to spend the rest of my Sunday bolting the device together so that it may be wall mounted (much to my wife's delight). The nice thing about this project is that it is all made from effectively scrap parts. That is to say, I didn't have to go out and buy anything, I just used what was lying around the house - with one exception, the transformer.

Now, you can't use just any old transformer for this caper. My second hand monster had a former life running a neon sign and I picked it up off eBay for £30 plus postage.

The transformer weighs a hefty 10kg and ramps up the 230V input to 10,000V at 50mA. Don't be fooled by that low-sounding current rating, this baby can kill ya, so it's not for the faint hearted. It turns out 10kV is the maximum voltage you'll find in a transformer of this type in the UK although other countries use higher voltage beasts which would allow for a greater gap between the electrodes.

Wiring of this project is pretty simple. The Live, Neutral and Earth connections go to the 230V terminals while each of the 10kV HT connections is wired to an electrode prong. The red lines indicate connectors which are in place on the lid of the transformer, so when the lid is removed, the Live and Neutral connections are isolated from the main inputs thus rendering the HT side safe. Clever.

One of the most tricky parts of the project is finding the ideal gap and shape of the electrodes. Too close together with not enough angle in the vee and the arc will remain at the base. Too far apart and you'll get no arc at all; just the scary buzzing of high voltage electricity to remind you that the electrodes are not to be touched!. Of course, you can't adjust the gap while it's live, so you have to be prepared to keep isolating the electrodes, making adjustments and retesting. Another site linked to here describes using resistors to form a bridge for the arc thus making the gap distance less critical but I followed the trial and error method and managed to get it working reliably.

With the prototype working, it was time to work on the final construction. When buying the transformer, I intended to make a portable unit, but once it arrived I realised the size and weight was going to make portability impractical. Instead, I chose to make it a wall mounted feature and as I had a hangover and didn't fancy going out shopping for parts, I scrounged around the house for suitable bits.

I had already found, and was using, my first parts - the electrodes. I had planned on making these out of a metal coat hanger or something, however rooting about in a kitchen cupboard I came across some metal barbecue skewars which were ideal. The (long suffering) wife wasn't too pleased about me having them but I explained that it was 'for science' and I think I got away with it. The next handy part was one strangely named 'modesty block' as used in joinery. The holes were ideal for keeping the skewars at a distance and there was a screw hole through the middle that I could use to attach it to a backboard. It's plastic construction made it an ideal insulator to hold the electrodes.

The arc is white hot, so using a combustable material such as wood or plastic for a backboard is not an option. What I did find however was a ceramic tile left over from when I retiled my toilet! These are large tiles and, being non conductive and resistant to heat, would make a perfect backboard. The only problem was that these tiles were a bugger to drill. My special (brand new) ceramic tile drill-bit shattered by the time I drilled the third hole and the three holes I did drill took a total of over an hour. I don't know what material runs through the centre of these particular tiles, but it was a real effort to get through. I had planned to have more holes for mounting a glass front and running the cables but with the effort required, loss of the drill bit and the worry that the tile might shatter if I was too hard on it, I decided to quit while I was ahead.

The above pic shows the modesty block shonkily bolted to the tile. Only one hole has been drilled at this point. The shape of the vee and outward curl at the top to break the arc is clear to see. The twists part way down each skewar is just the way the skewars were made and weren't put in by me. In fact you can see the poor quality of my metalwork skills in the frankly amateurish way the bottom ends of the skewars have been pulled out of the loop shape they were in and folded back on themselves around the modesty block so that the vee holds its shape. The arc has a mechanical effect on the electrodes and will push them apart as it travels so it is important that they are secured and that the vee retains its shape along its full length. The modesty block holds the electrodes a few milimetres above the tile surface so they are not resting on the tile itself. Notice that one of my electrodes is shorter than the other. I'm not sure why I made it end up like that - I think I just got it to a point where it was working and I couldn't be bothered to fiddle with the shape of the vee any further.

This close up shows some later work on the electrode base. I wanted to use some kind of physical connection to attach the wires but didn't have anything to hand. In the picture some rough solder joints are being used so that testing can take place during construction. These joints were later cleaned up with some proper soldering. Notice the black cube at the bottom of the modesty block. This is acting as an insulator to prevent a spark forming at the bottom on the underside of the block. This shouldn't happen as the arc will always take the easiest path between the electrodes which in this case is the <1cm gap above the block, however if one of the electrodes got bent or moved an arc can form on the underside if the gap shrinks too much. This did happen with my prototype in testing, the result being a rather burned modesty block and a bad smell. The black cube is a Lego brick I nicked from the kids' bedroom and is glued in place providing a physical insulator on the underside of the block.

With the electrode base mounted on the tile and the wires fixed in place, all that remained was to cover the unsightly gubbins and to fit a shield to keep curious fingers away from the electrodes.

To cover the base and wiring I rifled through some drawers and found a long unused cheap metal picture frame. Having removed the backing and the clear plastic windows, I was left with a fairly attractive looking piece of curved metal with three large square holes. I covered the holes from behind with some textured aluminium plating and bolted the metal frame to the tile using the remaining two holes that had been drilled. Finally I bolted two plastic A4 paper spine binders to the sides of the metal frame and slid a toughened glass sheet that was once a shelf in a kitchen cupboard into the spine binders. I was fairly pleased with the overall look, although I would have preferred the spine binders to have been silver rather than black.

The final job was to wall mount the whole assembly and connect it all up. I bolted it to the wall and ran the supply cable through a short run of existing trunking to the transformer which sits on the floor, this is then cabled to a switched circuit. This means two switches located in different areas of the room need to be activated in order to start the transformer. That should prevent the bloody kids from turning it on (they'll have to find the switches first). That said, I might introduce some other form of activation such as a keyswitch just in case they try to get clever. This unit is very dangerous and must only be used with correct supervision. The metal frame isn't Earthed as only the transformer HT is cabled to the unit itself and I wouldn't want the arc to jump to an Earth point instead of across the electrodes! Besides nobody sensible would want to get too close to it while it's on! Competant supervision should ensure onlookers look but don't touch (treat it like a firework).