As the flux in the ring is increased from 0 to $
icefrac{1}{2}Phi_{0}$ the magnitude of $ i^{-}$ increases to a maximum. As the flux is increased greater than $
icefrac{1}{2}Phi_{0}$ it is now energetically favourable for a current, $ i^{+}$, to flow in a clockwise direction, decreasing in magnitude to 0 as the flux reaches $ Phi_{0}$. The circulating current has a periodic dependence on the magnitude of the applied field, with a period of variation of $ Phi_{0}$, a very small amount of magnetic flux. Detecting this circulating current enables the use of a SQUID as a magnetometer.