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Our Jacobs Ladder was made from fairly easily obtainable parts including a power supply from a (large) neon sign.

Ladder pic 1Ladder pic 2

The pics above show our completed Jacobs Ladder with the 10,000 volt F.A.R.T (thats the acronym of the manufacturer and no indication of the contents of the case) transformer we purchased several months ago mounted on the base.

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WARNING

Building any device which uses a high voltage power supply such as a neon sign transformer is dangerous. You should only attempt it if you are fully aware of the dangers of high voltage electronics.

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Construction Notes

The most important thing you need to build a small Jacobs ladder is a high voltage current limited power supply.

We used a 10,000 volt neon tube power supply purchased second hand from a local neon sign maker / installer. Generally speaking, the higher the voltage the better as this dictates the maximum distance the spark will jump. Current limiting is also important because if you used a non current limiting supply, it would burn out / fuse out very quickly.

Neon sign transformers have both the high voltage required and built in current limiting. NST's are by far the best option for your first Jacobs ladder.

As we are based in the UK, unfortunately 10,000 volts is the legal limit for NST's (15,000 volt limit for the USA). For a more impressive / insane ladder with bigger arcs, a larger custom built power supply would be required.

All of the materials to build this device except the NST can be purchased from B&Q Warehouse (in the UK).

Actual Construction

The rods are 4mm steel and are bent into shape and welded to flat steel supports which are in turn bolted to the insulating base (which is made from a short length of plastic tumble dryer ducting). The ducting is then bolted to the wooden base. The base the rods are attached to has to be a very good insulator for obvious reasons.

The NST is mounted on the wooden base close to the rods and connected to the ends of the rods with terminal block connectors (with the plastic surround removed). The cable used needs to be well insulated (preferably HV cable). For neatness, the cable runs inside the ducting where possible.

The most critical part of building the Jacobs ladder is getting the rod gaps right and using an additional electrode at the bottom of the rods to help start the spark in the first place. We got the excellent idea for the electrode from the ladder that Big Clive built.

Ladder closeup The extra electrode which enables the spark to start automatically and for the rod gap to be easy to set is shown in the pic on the left. 

The electrode consists of a small length of thick copper wire connected to one of the rods via two 1 Megohm half watt resistors. The end of the copper wire is positioned just between the rods at the closest point.

The gap between the rods at the bottom of the ladder is 9mm. The length of the rods is 89cm from the bend in the picture above to the top where they bend outwards. The bend outwards at the top causes the spark to be pulled apart and the process to start again.

The rods have to be in a fairly tight 'V' shape for this ladder as the arc can only be pulled out so much before it will stop. The gap at the top of the rods before the outward bend is only about 16mm.

Ladder prototypes

Three of the previous prototypes are shown here. These previous attempts either did not start arcing initially or the spark did not travel the full height of the rods.

The ladder working

One (not very good) pic of the ladder working...

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