A small permanent magnet is always liable to become demagnetized, or have its polarity reversed by the action of lightning.
These electric pulses were made to act on an electromagnet at the receiving station, which, in accordance with Page's discovery, gave out a sound of a pitch corresponding to the number of times it was magnetized or demagnetized per second.
If it is desired to annihilate the hysteretic effects of previous magnetization and restore the metal to its original condition; it may be demagnetized by reversals.
We have observed that Nickel thin films may be partially demagnetized on time scales shorter than 300 fs.