Restoration of a Ruhmkorff Induction Coil
Ruhmkorff coil, an electromagnetic SteamPunk extravaganza
I recently was given a Ruhmkorff coil, an induction coil, I already had a small one but this one is a monster and I cannot thank the people that gave me this massive beast enough. I won't say who they are because you never know they might get into trouble.
The label says Philip Harris, Birmingham and it probably dates from the 1950s.
The machine was broken, maybe for the last ten years and has probably spent the decades since
Gagarin went onto orbit sitting on the bench and brought down occasionally to scare 14 year olds.
Certainly it was dirty.
Bits were missing and it had amazing grime, dust and corrosion.
I cannot find many decent picture of bits of kit like this on the interweb and certainly no pictures of this model
I took the bottom off and checked the connections. The large black object is a plate capacitor, marked that it has a capacity of 2 microfarads. As I looked at it, almost literally, the rubber insulation which had perished long ago fell off, so I replaced it. The other connections were of solid copper insulated with lacquer.
The switch is a type called a Ruhmkorff, not doubt after the good doctor. The connections go from the ends to the middle sections so if it is thrown one way the polarity is one way but can be reversed by pushing the switch to the other position.
When I had done a quick clean up on the contacts I put some copper wire onto the terminals and tried to see if the coil was alive. At first I used a bench power supply, I could get the coil to make small sparks but they would not sustain for long. I worked out it could be that the supply was at fault, unlike true DC, a bench supply is rectified from AC and so it is rather wavy.
The coil works by having a circuit that has a current flow through a primary coil with a solid iron core and this makes a magnetic field. The field causes a contact to be broken and the current stops flowing, field collapses, a spring on the contact moves it to remake the circuit and the current flows again. The collapsing and regrowing magnetic field does what they do and induces a current to flow in the secondary coil. The primary coil has only a few turns of wire and judging by the length of the sparks produced by the secondary coil it has to a good couple of hundred turns, possibly over a thousand and produces a couple of hundred thousand volts.
It takes DC and turns it into high voltage AC, I would not like to quote the current but I have no urge to put my hand in the way to guess the flow but giving your self a shock was an old fashioned way of working it out, a modern multimeter would probably not survive either.
I used a battery, one from a very excellent model car I have recently built and the beast growled into life.
Having made sure that it was worth the effort I carried on cleaning and remaking.
The actual coils were coated in wax and then had a bakelite type plastic covering, sadly broken and very fragile so in the interests of safety I replaced them. But with what? What had a got to hand? Leather, from off cuts I had had lying around for ages. Looks a bit like elephant hide, but I don't think it is. A non-vegetarian inductive machine.
For cleaning the brass I used a wire brush, then wire wool followed by toothpaste type metal polish and a lot of elbow grease. The woodwork I cleaned with meths and then careful use of boiled linseed oil onto the most oxidised areas.
As I said above, I could find very few pictures of these machines on the interweb, and I could not find any that were the same as this one, so since bits were missing and broken I was able to use some license and imagination, but did not do anything that could not be removed in the future.
The electrodes posts were missing, both broken off, but I had bits of one. I wondered what to use and decided on balls of brass and chair legs.
Cut down and with ball fitted.
To thread the bottom bolt, I used polymorph - the thermo softening plastic - in the hole to make the size up and I did the same with the brass ball. The balls were taken off and the new pylons stained to match the original and then varnished to a similar finish.
The balls had to have holes bored in them and another bored and tapped to take electrodes and locking nut. Luckily I have a lathe, only a very small one but just about big enough.
The other bits are accessories that I made to go with it.
Complete and nice and shiny
The standard electrodes
The original end cover was missing and it looked a little untidy and unfinished but I had a matching piece of bakelite from an old film projector, the copper rings were part of the switch mechanism and it fitted so I used that and a brass ball with the thread made up with polymorph as an end.
Switch and interrupter
Notice the little plaque which clearly says 6V only. Does not sound like a lot does it. But look what it can produce.
Standard electrodes. Notice the arc is stronger at one side than the other, considering it is AC I was surprised by the apparent polarity.
None of these pictures really do the machine justice, but strangely the video