As shown in Figure 8.49A, a super-conducting input coil must be used to couple the flux to be measured into the SQUID toroid. It appears that optimum SQUID sensitivity occurs when the inductance of the pickup loop, L P , is equal to that of the coupling loop, L C . Assuming noiseless flux coupling, Clarke and Koch (1988) showed that the equivalent flux density noise of the SQUID can be given by