A thin non-conducting ring of mass m , radius  a, carrying a charge q  can rotate freely about its own axis which is vertical. At the initial moment the ring was at rest and no magnetic field was present. At instant t = 0, a uniform magnetic field is switched on which is vertically downwards and increase with time according to the law B=B0t . Neglecting magnetism induced due to rotational motion of the ring. The angular acceleration of the ring as seen from above is  B0qxm , then x is______