Magnetized metals when cooled to the temperature of liquid air.
The metal, having first been uniformly tempered glasshard, should be annealed in steam at loo° C. for twenty or thirty hours; it should then be magnetized to saturation, and finally " aged " by a second immersion in steam for about five hours.
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.
The other phenomenon is mentioned by Greek and Roman writers of the 1st century: a piece of iron, when brought into contact with a magnet, or even held near one, itself becomes " inductively " magnetized, and acquires the power of lifting iron.
The availability of the energy of magnetization is limited by the coercive force of the magnetized material, in virtue of which any change in the intensity of magnetization is accompanied by the production of heat.