The Primary Coil forms is the inductive part of the primary circuit, along with the primary capacitor these two componants form a resonant circuit.
As the primary coil handles all of the stored power in the primary capacitance and is almost a short circut, It has to be capable of withstanding large current pulses. For this reason it is usallly constructed from copper tubing or heavy guage wire.
For Tuning purposes the primary coil is tapped to allow its inductance to be changed and therefore the resonant frequency of the primary circuit as a whole.
It is generally easier to vary the inductance than the size of the primary capacitor.
| Spiral wound Primary made from coper tubing
Lower coupling to secondary, Smaller Inductance per turn.
wound Primary made from copper wire.
High coupling, Larger inductance/turn