flows in the opposite direction, opposing the difference between the admitted flux and the external field of just over . The current decreases as the external field is increased, is zero when the flux is exactly , and again reverses direction as the external field is further increased. Thus, the current changes direction periodically, every time the flux increases by additional half-integer multiple of , with a change at maximum amperage every half-plus-integer multiple of and at zero amps every integer multiple. If the input current is more than , then the SQUID always operates in the resistive mode. The voltage, in this case, is thus a function of the applied magnetic field and the period equal to . Since the current-voltage characteristic of the DC SQUID is hysteretic, a shunt resistance, is connected across the junction to eliminate the hysteresis (in the case of copper oxide based high-temperature superconductors the junction's own intrinsic resistance is usually sufficient). The screening current is the applied flux divided by the self-inductance of the ring. Thus can be estimated as the function of (flux to voltage converter) as follows: , where is the self inductance of the superconducting ring The discussion in this Section assumed perfect flux quantization in the loop. However, this is only true for big loops with a large self-inductance. According to the relations, given above, this implies also small current and voltage variations. In practice the self-inductance of the loop is not so large. The general case can be evaluated by introducing a parameter with the critical current of the SQUID. Usually is of order one. RF SQUID The RF SQUID was invented in 1965 by Robert Jaklevic, John J. Lambe, Arnold Silver, and James Edward Zimmerman at Ford. It is based on the AC Josephson effect and uses only one Josephson junction. It is less sensitive compared to DC SQUID but is cheaper and easier to manufacture in smaller quantities. Most fundamental measurements in biomagnetism, even of extremely small signals, have been made using RF SQUIDS. The RF SQUID is inductively coupled to a resonant tank circuit. Depending on the external magnetic field, as the SQUID operates in the resistive mode, the effective inductance of the tank circuit changes, thus changing the resonant frequency of the tank circuit. These frequency measurements can be easily taken, and thus the losses which appear as the voltage across the load resistor