When current flows, a magnetic field exists around the wire. When the wire is wound into a coil, the fields add up and the field of one turn cuts across all other turns. The field in air is rather weak, the presence of iron in the coil magnifies the field and makes it stronger, but the ability of iron to increase the magnetic field is limited, at some point the increase of current does not cause an increase in magnetic field and the core is said to be saturated. An air core coil cannot saturate.
The magnetic field expands and contracts as the current increases and decreases, but the changing field induces a voltage in the coil that opposes the change in current. This is in accordance with Newton's third law: For every action, there is an equal and opposite reaction. This is the essence of induction. If a constant voltage is applied to a coil, the current will increase in a linear manner depending on the resistance and inductance, due to the back EMF (EMF=ElectroMotive Force = voltage)
At one point, I mentioned saturation of the PNP transistor: that is when the collector-emitter voltage reaches it's lowest value and cannot go lower.
The PNP switch is controlled by the inductance and capacitance. The key switch is just an on-off switch, I think this is a code practice oscillator. The inductance causes the current to increase slowly (linearly) and the capacitor keeps the PNP off while it is discharged by the bias resistors.
Inductance is a measure of the energy that can be stored in the magnetic field : E = L*I^2 where E is the energy in Joules, I is the current in amps. Inductive reactance is a measure of the impedance the coil presents to sine wave current. Note that reactance is only valid for sine waves. The symbol for reactance is X, the symbol for inductive reactance is Xl. There is a voltage drop across an inductor , like Ohm's law: V = i*Xl, but there is no power lost in the inductive reactance . A coil may get hot due to losses in the iron core or resistance, but not from the reactance.
Thats the lesson for today!