[nickdothutton]

I have occasionally wondered if a tape recording (audio, VHS, etc) would capture something of the earths magnetic field at the time and position of the information was committed to the media, and if this could be discerned.

[cynwoody]

It's not useful for mag tape or wire recordings, but archaeologists have available a technique that can date artifacts like campfires, pottery kilns, and burned out adobe houses within the last 10,000 years.

It's called archaeomagnetic dating[0]. It turns out that heated ferromagnetic materials, such as magnetite, capture the magnitude and direction of the earth's magnetic field as they cool down through the Curie temperature[1]. That allows an investigator to ascertain the direction of magnetic north the last time a likely sample was heated above the Curie point. Over time, magnetic north changes with respect to true north.

    [0]https://en.wikipedia.org/wiki/Paleomagnetism
    [1]https://en.wikipedia.org/wiki/Curie_temperature