Measuring the Resistance for DC

    To measure the resistance in a superconductor with DC current, start a current flowing in a closed superconducting ring and then see if there is any decay in the current after a long period of time. Unfortunately, there is no way of directly measuring the current without having an effect on the circuit itself. That is why in order to measure the current flow, the magnetic field that the circulating current produces and see if it decays with time; this is done because the measurement of the magnetic field does not draw energy from the circuit.


  Quinn and Ittner found that in this experiment, the magnetic field decayed by less than two percent in seven hours. From this they were able to calculate the resistivity of the superconducting metal at less than 4e-25 ohmmeters; the decay time exceeds 100,000 years. The metal that an electron travels through is modeled as a three-dimensional ion lattice with freely moving electrons.


    Because there is no resistance in the circuit, there is no electric field. There is no electric field because in DC, the electrons are not accelerating. By Faraday’s Law, this means that there cannot be a change in the magnetic flux in this type of circuit.


The Resistance under AC Conditions

    If the current in the superconductor is changing, or AC, an electric field must be present in the superconductor to accelerate the electrons. This electric field is caused by electrons moving perpendicular to the flow of the rest of the current. These electrons then collide the other electrons and this creates resistance. Therefore, AC current in a superconductor creates resistance. Zero resistance can only occur with DC current, which does not require an electric field.

Title Page
Introduction to Superconductors
Basic Conditions
The Resistance in a Superconductor
The Two Types of Superconductors
Applications of Superconductors