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Pirelli’s engineers realised that these defensive weapons are the ideal size to add to tyres in order to control hysteresis loss, and that a ready supply of them is available in the husks left over from the milling of rice.

Hysteresis loss, as this is known, can be reduced by mixing a tyre’s rubber with powdered material that has strong chemical bonds in it.

Silica is better because it lowers hysteresis loss without reducing a tyre’s grip.

For most purposes, however, where a laminated iron magnet core is required, the flux density is not pressed up above 6000 units, and it is then more important to secure small hysteresis loss than high permeability.

The testing of sheet iron and steel for magnetic hysteresis loss has developed into an important factory process, giving as it does a means of ascertaining the suitability of the metal for use in the manufacture of transformers and cores of alternating-current electromagnets.