This is a question about magnetic field strengths. The story goes that someone carries an important audio master (magnetic) tape on a UK mainline train, only to find that it has erased itself! Horror!
I think it's FALSE. Here's my working:
I'd guess the notion is that it's the field generated by the overhead power cable, which is at thousands of volts and drives at least hundreds of amps. (1000 ton train to 50m/s in 5 mins needs 5MW, at 25kV, that's 200A during accn, much less when cruising, I'd guess. Add efficiency losses to taste).
I think the read/write head of tape deck might be about 1500 Oe (from Erasing Myths About Magnetic Media by Sidney B. Geller, 1976, p"68" (PDF)), which in non-magnetic materials is about 1500 gauss, or 0.15T. (Really? Wow!)
And it's applied at point-blank range to the tape - say < 1mm, but only for a moment. Tapes run at a few cm/s, say 10cm/s, so that 1mm spot only gets 10ms of attention, though Geller indicates that exposure time is not a factor: peak field strength is. I think magnetic fields reduce as the cube of distance in air. Finally, again according to Geller, the erasure has to overcome the "coercivity" of the mag-tape, which in data-processing tapes of the 1970s, was apparently ~250 Oe - much less than that, and you don't disturb the encoding. Not sure how that would compare to professional audio tape.
Suppose you put the tape in a case on an overhead luggage rack, and it's only one metre from the external high current power wire. And there's no train roof to weaken the field. According to this calculator, I'd need 500kA at 1m to get 0.1T. So, a thousand times more than actual current, even with no big steel roof in the way.
Or are there other sources of intense magnetic field on trains that I'm ignoring? The engines are all at one end. The brakes...? Perhaps a passenger gets on with a fully operational MRI machine and puts it one metre away. It still needs to be a billion times more powerful than the tape head, 100MT. That'd be like a magnetar or something - ok, there's something wrong with my sums here.
I haven't done magnetics calculations since I was a teenager. I fear I'm carelessly jumping between field strength and flux, and I'm unconfident of converting between oersteds and teslas (or gauss). How much of the above makes any sense at all?
