In calculating the integral that one evaluates in applying the Biot-Savart Law one is

Answer:

The Biot-Savart Law itself written in discrepancy form gives the infinitesimal magnetic field at a point calls it point P due to an infinitesimal element of current. For example if the magnetic field is due to a current in a thin wire then the infinitesimal magnetic field in question is due to a specified infinitesimal length of the wire. In computing the total magnetic field at point P we add up the contributions from each and every infinitesimal length of the wire. We are therefore integrating (summing) over the wire.