Originally Posted by s/v Jedi
So, I step back to Wotname's post: a grounded pointed rod or brush is proven to dissipate an electric charge. The effect was discovered by Benjamin Franklin so not new tech at all. I still don't understand why the dissipators on planes have an electric resistance and I can't find any info on that on-line. The only info I find says they are connected to the fuselage. But that connection can have a 50- or 100kOhm resistance or none at all. I tend to believe the guys that work with/on them but don't understand the theory behind it.
I can't lay my hands on the manufacturers data right now for static wicks but will post it when I find it - assuming this thread is still topical.
Here is how I think it works (but remember I am not a boffin, just an avionics field engineer
so I might be off track).
A static build up by definition means a EMF (or a PD) is created between two points seperated by an insulating material - in this case air.
As the insulator (air) prevents current
flow, the PD continues to increase until there are enough volts to ionize the air gap. At sea level, this will need to be in the order of 30+ thousand volts to ionize a small gap of some millimetres and 100's of thousand of volts if the gap is much bigger.
Once ionized, the air is no longer an insulator but becomes a rather good conductor so by Ohms law, large current
flows. I.E. Big volts and low resistance equals big current.
A low resistance static wick (or brush) simply moves one end of the circuit to a new point in space, i.e. one pole of PD is now at the end of the brush instead of the end of the mast
. Kirchoff's law IIRC.
However, a static wick with say 100Kohms changes the circuit substantially. Now we still have big volts, low resistance ionized air AND a high resistance wick in series so current flow is substantially decreased. Therefore EMI / RFI is reduced due to reduced current flow and the charge (PD) takes longer to discharge as the current flow is lower.
Another way to say it is: the spark is smaller.
I am sure someone could do the maths of the time constants etc of the circuit but I have a boat to rebuild
However, just thinking aloud, I believe (but don't know) that the static charge felt on the boat during a thunderstorm should be considered as "resultant" static charge. By this I mean, the boat had nothing to do with the creation of the static charge, it was just floating in the water that has charge "impressed" on it by the movement of water and air molecules moving around in the thunderstorm clouds. Therefore trying to discharge this PD by a rod or brush on the mast seems pointless to me as the amount of charge is going to be BIG.
The static wick on the aircraft is trying to create a small controlled spark to discharge the airframe to the surrounding air rather than having a bigger uncontrolled spark discharging the airframe. I can't see how one can create a small controlled spark on the mast brush to discharge a large "resultant" charge of the surrounding water.
Lighting is just a big (very big) spark and follows all the rules of physics, the only thing is that we don't know all the physics of electrical
current flow and ionization.
At the risk of further thread drift, it is impossible to create a spark with a PD of 12 or 24 volts, for those who don't believe this, PM me for the explanation or I can start another thread about this (if anyone is interested) rather than drift this thread any further
. Hint, you can use 12 volts to create a very large voltage which will create a spark.