# Bird on a Wire Revisited **URL:** \ **Category:** Factual Questions\ **Created:** [May 16, 2009, 12:40pm UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636 "2009-05-16T12:40:45Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 16, 2009, 11:07pm UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/21 "2009-05-16T23:07:31Z")
> [@](#): > > That’s easy: No, because grabbing onto it would surely yank me right out of the basket. Or alternately, No, because I have no good reason to grab it, and I try to stay in the habit of not grabbing high-voltage wires. ---
**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\ **Post date:** [May 16, 2009, 11:13pm UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/22 "2009-05-16T23:13:52Z")
> [@HongKongFooey](#): > > Your question is more about how the bird gets to the wire without being shocked than how it sits on the conductor, right? I think it’s a good question. If you watch a high voltage inspection where they tie the helicopter to the high voltage line to get them to the same potential there is an arc when they use the shock stick to reach out and touch the line at the beginning of the procedure. This is because helicopter rotors produce hella static charge. In fact, if you are ever in need of helicopter rescue, don’t reach out and grab the cable until it hits the ground or you will receive a nasty surprise. I don’t think birds have the same problem. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 16, 2009, 11:22pm UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/23 "2009-05-16T23:22:53Z")
Are you saying, **beowulff** , that if I stretch a conductor out and connect it to a transformer at 12kv, current will flow back and forth at 120 times per second even at no load? The potential would go +60v then -60v, but there would be no flow. If that was a 480v line, and I clamped an ammeter around it, I would read zero amps. I’m not argueing. Something is missing here, and I wonder what it is. If that 12 kv wire had a high resistance, higher that my body, and I grabbed it with both hands at different points on that wire, current would definitely flow through me. This effect is similar to [step potential](http://industry.flexiblelearning.net.au/learning_objects/GippsTAFE_e_learning_siteV4/Pages/examples3.html), as seen in this cute animation. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [May 16, 2009, 11:49pm UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/24 "2009-05-16T23:49:24Z")
I might be wrong but I think he meant a person touching the line wouldn’t change potential across their entire body instantaneously as the wire changes potential so there would have to be a sort of charging current for lack of a better term as they charged and discharged. I’m used to being wrong though. 😃 ---
**Author:** ![Biffy\_the\_Elephant\_Shrew](https://avatars.discourse-cdn.com/v4/letter/b/e47c2d/32.png) [@Biffy\_the\_Elephant\_Shrew](https://boards.straightdope.com/u/Biffy_the_Elephant_Shrew)\ **Post date:** [May 17, 2009, 12:01am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/25 "2009-05-17T00:01:01Z")
So when will we revisit Drunk in a Midnight Choir? ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [May 17, 2009, 12:02am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/26 "2009-05-17T00:02:15Z")
I vaguely remember that you could set up equations for capacitors of arbitrary shape, but my physics is far too rusty and I don’t even have my old textbooks to look at. If you assume that the bird is a parallel plate capacitor (in the same magic physics land that cows are spherical…), you can calculate a capacitance. For the hell of it, assume the largest possible bird, the Andean Condor (wing/plate area of 1.5 m^2), on a really low power line (3 M above ground), that happens to be really high voltage (100 kV). This should give an absolute upper bound of how big the capacitance and currents could be. This [calculator](http://www.daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml) gives the bird a 4.43 pF capacitance. The bird-capacitor will be charging from -100 kV to 100 kV, and back again, 60 times per second. 200 kV on a 4 pF capacitor gives a charge of 443 nC. Now, assuming constant current from peak to peak (essentially a sawtooth wave), 443 nC/(1/120 s) = 53 microamps. Which is pretty much nothing. I don’t think anyone can feel anything less than a milliamp directly. And that’s a theoretical maximum. A smaller bird, on a higher wire, at lower voltage, and non-zero resistance will see much smaller currents. A parallel-plate sparrow, on a 120V line 30 m above the ground will have currents on the order of a few picoamps. Adding the effects of resistance will reduce the current by another order of magnitude or more. I leave that as an exercise to the reader, since I’ve past the limit of physics that I comfortably remember. RC circuits, anyone? In other words, if you want to measure this, you’re going to need a pretty damn good oscilloscope. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [May 17, 2009, 12:23am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/27 "2009-05-17T00:23:42Z")
> [@lazybratsche](#): > > Either way, the current passing through the bird would be about three tenths of nothing. Looks like you’re right. Damn, I was going with the square root of fuck all. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 17, 2009, 12:30am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/28 "2009-05-17T00:30:11Z")
My memory of a capacitor is of two conductors seperated by a dielectric (insulator). Clean dry air is a much better insulator than the body, and might work as the dielectric, but where’s the other conductor? That’s kinda what lazybratsche said, ian’t it? ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [May 17, 2009, 12:36am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/29 "2009-05-17T00:36:15Z")
The HV cable – air – bird. The bird is the other ‘plate’ of the capacitor. It’s not a good conductor like the wire is but it can still hold a charge. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 17, 2009, 12:36am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/30 "2009-05-17T00:36:40Z")
> [@Biffy\_the\_Elephant\_Shrew](#): > > So when will we revisit Drunk in a Midnight Choir? You wanna pass that around, Bogie! ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [May 17, 2009, 12:39am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/31 "2009-05-17T00:39:36Z")
The other conductor here is quite literally the ground beneath the bird. The ground itself is what completes the circuit in power transmission. I think that even that’s not necessary, as objects floating in free space will still have a capacitance. ETA: Each side of the capacitor is directly connected to the power transmission circuit. The bird is directly connected to the wire it stands on through its feet. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 17, 2009, 12:44am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/32 "2009-05-17T00:44:32Z")
> [@HongKongFooey](#): > > The HV cable – air – bird. The bird is the other ‘plate’ of the capacitor. It’s not a good conductor like the wire is but it can still hold a charge. But the bird is “common” to the wire. It changes potential with the wire at light speed. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [May 17, 2009, 12:45am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/33 "2009-05-17T00:45:40Z")
Ahh, I thought you meant the bird in flight before it reaches the line. Isn’t that what we were trying to figure out? Me confoozled. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [May 17, 2009, 12:47am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/34 "2009-05-17T00:47:57Z")
I would like to point out that I have personally seen hummingbirds that have been killed by landing on the hot wire of an electric (pulsed) fence. They were not in contact with any other conductor. The high DV/DT of the pulser may make this a worse-case scenario, but it certainly shows that it’s possible to kill a small bird through capacitance effect alone. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 17, 2009, 12:59am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/35 "2009-05-17T00:59:50Z")
> [@lazybratsche](#): > > The ground itself is what completes the circuit in power transmission. I _think_ that’s a misconception. The circuit is completed by the other wire, or wires in a three-phase circuit. Even in household circuits the circuit is completed by the neutral wire, which is indeed grounded. But _not_ part of the circuit. The ground (actually “earth”) is a pretty poor conductor, and varies widely in resistance. Did I misunderstand your statement? ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 17, 2009, 1:27am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/36 "2009-05-17T01:27:22Z")
> [@beowulff](#): > > I would like to point out that I have personally seen hummingbirds that have been killed by landing on the hot wire of an electric (pulsed) fence. They were not in contact with any other conductor. The high DV/DT of the pulser may make this a worse-case scenario, but it certainly shows that it’s possible to kill a small bird through capacitance effect alone. I’d understand this if the bird were near an insulator and the post were at ground potential. Air is a good insulator, but if the potential is high enough, and/or the distance is close enough, current can flow. The bird, a relatively good conductor, could shorten the distance between hot and ground. I’m still pondering the capacitance idea. 😕 ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [May 17, 2009, 1:49am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/37 "2009-05-17T01:49:40Z")
> [@mangeorge](#): > > I’m still pondering the capacitance idea. Does it help if you think of the plate of the conductor as a charge collector rather than a conductor? The ground can store a lot of charge as does the wire. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [May 17, 2009, 2:16am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/38 "2009-05-17T02:16:36Z")
> [@mangeorge](#): > > I _think_ that’s a misconception. The circuit is completed by the other wire, or wires in a three-phase circuit. Even in household circuits the circuit is completed by the neutral wire, which is indeed grounded. But _not_ part of the circuit. > The ground (actually “earth”) is a pretty poor conductor, and varies widely in resistance. > Did I misunderstand your statement? I could be wrong here. As I said, this is right up against the limits of my physics knowledge. The vast majority of my hands-on experience is with electronics where the chassis is the ground and most definitely completes the circuit. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [May 17, 2009, 2:26am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/39 "2009-05-17T02:26:45Z")
> [@lazybratsche](#): > > I could be wrong here. As I said, this is right up against the limits of my physics knowledge. The vast majority of my hands-on experience is with electronics where the chassis is the ground and most definitely completes the circuit. Yeah, in power transmission there is a separate neutral or return line or separate phase conductors as **mangeorge** said. Sometimes they run overhead grounds for lightning strikes or safety purposes but the ground still acts as a capacitor plate just not a conductor in the circuit. It can become a conductor in the circuit if something goes wrong, that’s why things like [neutral ground resistors](http://www.avtron.com/neutral_grounding_resistors.htm) become necessary. ---
**Author:** ![mangeorge](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@mangeorge](https://boards.straightdope.com/u/mangeorge)\ **Post date:** [May 17, 2009, 2:39am UTC](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636/40 "2009-05-17T02:39:55Z")
> [@lazybratsche](#): > > I could be wrong here. As I said, this is right up against the limits of my physics knowledge. The vast majority of my hands-on experience is with electronics where the chassis is the ground and most definitely completes the circuit. Chassis ground, yes. Bad naming. It should be called “common”, imo. It may be connected to the grounding (green) plug wire. I don’t know. The other (more appropriately named) ground is earth ground. It’s used as a safety. Some other countries call it “earth”, which makes sense. Grounded conductor = neutral, usually white. Grounding conductor = safety ground, usually green. This has nothing to do with the OP. [Previous page](https://boards.straightdope.com/t/bird-on-a-wire-revisited/496636.md?page=1)