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The primary difference is the way in which the machines measure this parameter. The use of a tensile machine is the traditional method used to determine the plastic strain ratio (r value) where the degree of necking is physically measured; the Modul-r uses a more theoretical resonant frequency technique that employs a magntorestrictive oscillator; the magnetostrictive oscillator is simple feedback system consisting of a measuring head, an amplifier and an electronic counter. The measuring head consists of three coils that form the sample chamber. When the sample is inserted and the TEST button depressed, an alternating field is created by the current passing through the coil network, producing cyclical longitudinal expansion and contraction in the sample through a process called magnetostriction.