Most metals and alloys have a negative temperature coefficient of modulus of elasticity; that is to say they lose stiffness when heated. They also have a positive coefficient of thermal expansion, increasing in length when heated. These two effects are due to increase in energy of the atoms with increase in temperature. Some ferromagnetic materials, however, exhibit markedly different behavior, which can be utilized to design constant modulus alloys.

The modulus of elasticity € of ferromagnetic materials is a complex function of a number of physical properties, related by the adjacent equation.

Each of the factors ?, µ and k is affected by composition, strain and temperature. In order that the modulus of elasticity will remain constant with variation in temperature it is necessary to select a composition for which ?²µ changes at the same rate and in the same direction as k².

The same constants are also sensitive to applied magnetic fields so that the modulus of elasticity changes with a change in magnetic field strength.

As these considerations apply only to ferromagnetic materials they do not hold when an alloy has been heated beyond its Curie temperature, the point at which it changes from ferromagnetic to paramagnetic behavior.