Working of Mini Tesla coil.

https://www.youtube.com/watch?v=cSM2kaaWwsk

Components used :

1. Transistor =  PN2222A
2. Copper wire =  26gauge
3. 9V battery
4. Resistance =  22K





Here, the transistor is working as a switch which is continuously being reverse biased (off)  by the back EMF induced in coil 1 (primary with 3 turns). This is happening at high frequency.
 
when we close the switch current starts flowing through emitter to base(5%) collector(95%) .
The collector is connected to primary coil  and when current passes through it , it creates an induced EMF in secondary coil which in turn creates a back EMF in coil 1 and hence coil 1 makes the Collector -Base   reverse biased(off).

 This is (ON/OFF) is happening at high frequency.

Also, some amount of current is being fed to the secondary coil through Base but the upper end of this coil is left open  due to this ,the secondary coil creates a heavy voltage to throw it in air.

High voltage means strong electric field that is why whenever  a CFL comes in its electric field it starts glowing.

SOME INFORMATION
1 . The voltage created is as high as 870V ( nearly).
2. For larger tesla coil you can use transistor BD135



THANKYOU  :)