The reciprocity integral: Convolution or correlation?

In communications or filter theory there is very little chance of confusion between the operations of convolution and correlation because time flows "one way" only. In magnetic recording theory, phenomena occur in the spatial domain and the reciprocity integral is given by some authors as convolution and others as correlation. Although it is entirely arbitrary in the spatial domain which direction represents an increasing x coordinate, any inconsistency in the application of a convention can cause errors. Here the usual convention is adopted that movement or displacement to the right increases x. It is shown that the reciprocity integral is correlation between the recording medium magnetization and the head fringing field function.