How could a train affect my compass?

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Hilmar Steen

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How could a train affect my compass?
« on: 13/01/2011 05:30:05 »
Hilmar Steen asked the Naked Scientists:
Today, on my way to hill-hiking by the Hong Kong MTR subway, I checked my compass while waiting for the train.

I did not expect a correct North direction underground due to magnetic fields from steel and electric cabling. but was surprised to see the needle dancing from side to side when the train approached.

On the train I checked my compass again and the needle was moving.

It seemed that when the train accelerated the needle was steady, like it was locked in position, but then as the train approached full speed, the needle began to bounce from 90 degrees from side to side. When at full speed, the needle moved only a little, but as the train's speed reduced, the needle bounced a full 180 degrees, until eventually as the train was braking to stop at the station the needle did three full circles.

I tried to get explanation by searching the internet  - without success
and would like to know what was causing my compass to act this way.

What do you think?
« Last Edit: 13/01/2011 05:30:05 by _system »


Offline Geezer

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How could a train affect my compass?
« Reply #1 on: 13/01/2011 06:47:31 »
The HK MTR appears to use 1500 V DC on the live rail, so there will be some rather large fields induced by the supply to the trains. If the trains use retro braking, they will be sending current back into the system when they are stopping at a station. This could account for some of the field reversals you saw in the stations because the brakes would not be applied continuously, but more in short on/off bursts.

Things will get a lot more complicated when you are on the train, because there will be fields produced from the track as well as fields from the traction motors, but the steady state when it was accelerating is not a surprise as all the fields would be fairly stable in that situation. When it reached full speed, it's really coasting and not drawing much, if any, current at all, so your compass could be reacting to all sorts of weaker field changes, some of which might be simply caused by large steel objects beside the track.

The problem with a compass in this situation is that it only tells you about the direction of the field, but it does not tell you much about the strength of the field.
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Offline yor_on

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How could a train affect my compass?
« Reply #2 on: 13/01/2011 18:15:18 »
Wow, cool effect :)
And nice explanation too.
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