The gate may be biased in the "zero-voltage" state. The junction may be switched to the "non-zero" voltage state by one of two means:

- Increasing the gate current (Ig) above the critical (Icr) level.
- Decreasing the Icr level below the existing gate current level.

Generally, the superconductive state corresponds to a logic state of "0", the resistive state to a "1". The nonzero voltage level is essentially the threshold voltage Vg, which will range from about 1 to 3 mV for classical superconductors.

Vg is related to the superconductor energy gap (D) by

The symbol on the left is commonly used to represent the Josephson junction in circuit diagrams. On the right is the Stewart-McCumber equivalent circuit for a Josephson junction. The sinusoidal variation of current as a function of junction voltage V.

Superconducting inductors that must transport relatively high current densities in high magnetic field have a variety of applicatrons:

- Coils for windings in motors and generators (Utility, automotive, marine propulsion applications).
- High-field magnets for research applications (Particle accelerators, material research).
- Magnetic Levitating (MAGLEV) coils for high-speed ground transportation.
- Superconducting Magnetic Energy Storage (SMES)(Electric utilities, Military applications).
- Magnetic containment fields for thermonuclear fusion research.
- MHD (Magnetohydrodynamic) EMT (electromagnetic thrust) systems for marine propulsion applications.
- MRI (Magnetic Resonance Imaging) which requires extremely uniform magnetic fields at the 10-20 kgauss level (formerly known as NMR, nuclear magnetic resonance).

An eddy current may be induced in adjacent strands of superconductor that are embeded in a normally-conducting substrate. This current, if substantial, can lead to quenching of the superconducting state. The eddy current is reduced by a practice familiar to electrical engineers who wish to reduce external magnetic fields around lines carrying alternating currents, that is, twisting of the individual pairs of superconductors. The shorter, the twist length L, of course, the more effective the technique. An approximate formula for the induced eddy current Jec is:

Characteristic decay time t of the eddy currents after the source (B):

- The inherent inductive reactance of the armature is greatly reduced, resulting in improved dynamic machine perfomance and voltage regulation.
- Space for the armature winding is increased, which increases potential power density and generator efficiency.
- By elimination of the interleaved stator iron (at ground potential) insulation requirements on the armature are reduced and/or much higher voltage may be delivered at the armature terminals. Higher terminal voltage can eliminate the need for a step-up transformer.

It is important to note that superconducting generator, while benefiting enormously from the higher current densities that may be achieved with conventional superconductors, do not require the current densities that are commonly necessary for a variety of high-field electromagnet applications.

Another potential application for superconductors is in the construction of electromagnetic waveguides. The advantage over conventional metal waveguides would be at the higher frequencies. In the case of mm-sized waveguides, attenuation becomes prohibitive except for applications where the guide length is very short, that is, usually less than a meter. At mm wavwlengths, conventional metal guides have attenuations on the orther of 10 dB/m due the high value of surface resistance (Rs) of the metal walls at ~200 GHz.

In contrast, shielding of low-frequency (or especially dc) magnetic fields normally involves the use of relatively expensive (and sometimes very thick) magnetic materials. One may consider these materials to be "short-circuiting" the applied magnetic flux. The reduction of these low-frequency or dc fields to arbitrarily low values becomes essentially a problem of cost and, the mass of shielding that can be tolerated for a particular application.

The concepts of zero (dc) resistivity and the Meissner effect which have already been intraoduced are, of course, of criticall importance in the use of superconductors for electromagnetic shielding. Limiting factors in the use of superconductors for shielding applications involve the critical field (

Recall Ohm's law and Maxwell equation:

__AC Shielding:__

In the normal state, surface resistance is a function
of the skin depth (d) and conductivity (s)
of the conductor, that is,

__DC and Low-Frequency Shielding:__

A fixed amount of flux is trapped by the deflated shield, but when the shield is expanded (below Tc), the internal flux density is reduced due to the greater affective area of the expanded shield. This method can (theoretically) be repeated, that is, expanding shields within expanding shields, to eventually provide a net shield where no trapped lines will exist in the innermost shielded volume. By this method, fields have been decreased on the order of 500 per folded enclosure. Such configurations are also referred to in the literature as "bladders" and "baloons".