There are a number of ways to introduce tensors. We shall begin by considering tensors of zero, first and second rank, before extending our discussion to tensors of third and fourth rank.Scalar quantities are examples of tensors of zero rank; these simply have magnitude. Vectors are tensors of the first rank. These have both direction and magnitude and represent a definite physical quantity. Tensors of the second rank are quantities that relate two vectors.Suppose we wish to know the relationship between the electric field in a crystal, represented by the vector E, and the current density (i.e. current per unit area of cross-section perpendicular to the current), represented by the vector J. 1 In general, in a crystal the components of J referred to three mutually perpendicular axes (Ox 1 , Ox 2 , Ox 3 ), which we can call J 1 , J 2 and J 3 , will be related to the components of E, referred to the same set of axes in such a way that they each depend linearly on all three of the components E 1 , E 2 and E 3 . It is usual to write this in the following way: 2 3 .( )