Transistor 2N2222A (caution - transistor gets very hot)
You can use other transistor
The Tesla coil works due to presence of parasitic capacitance between secondary and ground
Initially a small current flows to the base of the transistor and a large current flows in the primary this Induces EMF in secondary as number of secondary turns and large as compare to primary therefore EMF induced is also a large this cancels out the base current coming from the source and thus the slayer circuit works
