1. The problem statement, all variables and given/known data An infinitely long hollow cylinder of radius a carries a constant current I. Use Biot-Savart's law and show that the magnetic field at the centre of the cylinder is zero. 2. Relevant equations Biot-Savart's law 3. The attempt at a solution I cannot seem to come up with anything meaningful. I have tried searching this question but it seems the best way is normally to use Ampere's law and not Biot-Savarts law. Could someone explain the steps or show the steps how we could use Biot-Savart's law in this case.