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I'm trying to understand the physics. The spark plug has a gap of 0.9mm. Why would the current go through the insulation resistance, the finger resistance and the air gap between the finger and the engine ground instead of traveling a much less resistance through the 0.9mm gap?

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    \$\begingroup\$ Cracked insulation? Deteriorated insulation? \$\endgroup\$ – winny Sep 8 at 21:16
  • \$\begingroup\$ If the spark plug has been splashed with salt water, the current might go round the insulator, not through it. \$\endgroup\$ – Brian Drummond Sep 8 at 21:17
  • \$\begingroup\$ I would have seen a crack if it had one. I touched the spark plug wire far from the end of it. Even if it had a crack, or lets say not insulated at all, I still don't get why the current will go over a large distance of air instead of going through the short distance at the designed air gap. This happened to me at least 3 times already, once with a motorcycle and twice with a car. \$\endgroup\$ – Ronen Festinger Sep 8 at 21:28
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    \$\begingroup\$ You'd think after the first time you'd have learned not to do that ;) seriously though...the others are probably right, its your body capacitance. You're probably looking at electricity as a flow of electrons and wondering how they pass through rubber. The thing is that's only the simplest flow. It it entirely possible to transmit electrical energy without any electrons actually bridging the gap. Thats what happens in a microwave oven or really any radio. If you want to learn more, google 'displacement current' as a start \$\endgroup\$ – Kyle B Sep 8 at 22:23
  • \$\begingroup\$ Did it feel like a really strong shock and cause an involuntary muscle reaction, or a comparatively mild zap? Did the engine hiccup as if the spark plug itself didn't fire for a cycle or two? \$\endgroup\$ – pericynthion Sep 8 at 23:26
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Your finger on the wire forms a capacitor to the plug wire center conductor; the wire sheath is the dielectric. This couples to your body, forming a DC path to your torso. So the AC-coupled high-tension charge is flowing in and out of your finger to your body.

How? Even your ungrounded body presents a non-trivial capacitance to ground through the surrounding air, and in between any insulator and car ground. There will be some current, like static electricity.

Now if you’re leaning on the fender, that makes a fairly big capacitance to car body ground. Given the voltages involved (~50kV for some systems), while you may not see a spark on your finger, you still might feel some current. Stand away from the fender and the body-to-car capacitance - and sensation you feel - will be less.

This across-the-insulator capacitive coupling between the plug lead and ground is why plug wires are routed away from the engine on insulating standoffs: to reduce shunt losses in the lead and instead deliver all the coil’s energy to the plug where you want it.

The underlying physics is called displacement current, which is a fundamental part of how capacitors work (electric fields straining electron orbits). A change in charge induced by voltage change is accompanied by a current into and out of the capacitor as the charge changes. There doesn’t need to be spark for that to happen, current still flows.

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  • \$\begingroup\$ And the spark isn't "suddenly just there." It takes some microseconds to form, during which time the voltage across the gap is increasing to several thousand volts. This is what you are feeling - capacitive coupling to your finger of the time between when the ignition fires, to when the arc is created. \$\endgroup\$ – rdtsc Sep 8 at 21:47
  • \$\begingroup\$ So you are saying that because the body has less resistance than the air, putting the hand between engine case and the wire will cause a current path in addition to the spark plug gap? This would mean that it will shock you even if you don't touch the wire, also wouldn't you see sparks in the air? How far could it travel through the air anyway? \$\endgroup\$ – Ronen Festinger Sep 8 at 21:51
  • \$\begingroup\$ Yes, basically: body has lower resistance than air, forming a shunt path to ground. The shunt current varies with all the series elements. Wider separation means lower capacitance and less current. \$\endgroup\$ – hacktastical Sep 9 at 5:53

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