coilelectromagnetismsuperconductors
Superconductors are strong diamagnetic in nature so if they are used as core of any coil do magnetic lines of forces pass through them – I mean inside them just as with a soft iron core?
Secondly if they are used as the coil wound on core then change in magnetic field induces current inside them. Does mutual inductance occur in them or not – because the magnetic lines of forces don't pass through them, or because they bend towards them?
Audio inputs are normally AC coupled (there is usually a series capacitor to block any DC component in the input). Typically this means you won't see much below about 20 Hz and you certainly won't be able to measure DC or slowly varying signals.
As for sensitivity, typical "live level" inputs expect a signal of 775 mV RMS which corresponds to 0 dBV. Microphone inputs are usually more sensitive than this, but there is no "standard" sensitivity and the input hardware often has some kind of controllable gain stage prior to the A-D converter.
Here you will find a good explanation using a PNP transistor instead of a NPN, but the way it works is the same. In what follows I will describe the general idea.
To understand how the feedback occurs look at the picture below. The red U is the fork (for illustrative purpose). Each leg of the fork has a permanent magnet that interacts with the drive coils and the feedback coils. When it is oscillating around its equilibrium position, the maximum speed will be achieved when the legs are passing through that equilibrium position, like a mass on a spring. It is during maximum speed that the induction will be maximum.
Now suppose that the capacitor is initially discharged, then the voltage from Vdc will bring the base voltage above the emitter voltage and the transistor will turn on. The current that will flow through the drive coils D1 and D2 will be enough to start the oscillations. Meanwhile, the base current will start to charge the capacitor. In steady state the voltage induced in the feedback coil F1 will be turning the transistor on and off, with the capacitor barely discharging during the off cycles (C*R >> 1/(frequency of oscillations ).
Suppose the the fork is going through the point of equilibrium, then it will induce a voltage in F1, with the sign depending on the direction of movement. Only one of those peaks of voltage will turn the transistor on, the one which brings the base voltage above the emitter's. When that happens the current through D1 and D2 will sustain the fork oscillation by creating a magnetic field that 'pushes' the permanent magnets in the fork.
Next, suppose that the oscillations grow larger in amplitude and let's see how the circuit corrects it. First, a higher voltage will be induced in F1, but that shouldn't matter when the transistor is fully on (saturated). On the other hand, a higher voltage will also be induced in D1 and D2 and it will be such that it will tend to oppose the current drawn by the transistor through the collector. The result is that the driving current (collector current) decreases, and that means a smaller magnetic field and consequently a smaller "push" to the fork. Therefore the oscillation amplitude will decrease.
Another way by which the circuit corrects itself is through the capacitor C. If the oscillations get larger then a higher voltage will be induced in F1, but if the transistor is not fully saturated then a higher current will flow through the base in response to the higher voltage in the base with respect to the emitter. That current will charge C during the ON cycle, which will begin to discharge during the OFF cycle through R. So a higher base current means a higher voltage in the capacitor at the end of the ON cycle, which means that it will be harder for the next peak of voltage to overcome that new capacitor voltage and turn on the transistor in the next cycle. Remember the RC constant is relatively big, so the charge time of C is larger than the discharge time.
I think that's all. If you are wondering how a vibrating fork could replace the escapement of a mechanical watch watch this video.
Best Answer
As superconductors are perfectly diamagnetic, I would describe putting them in a transformer core as misuse, rather than use. They will do exactly what you don't want in a core material. Soft iron allows many times more flux for any given field than air, a superconductor allows zero flux, regardless of the field.
Superconducting windings, coils, behave just like ordinary windings, as far as having voltages induced on them due to changing magnetic flux linkage, and generating a field due to current flow through them, but without the \$I^2R\$ heat.