Mount the transistor onto the heat sink. The heatsink is important as the transistor gets hot. I just bought the cheapest heatsink maplin sell. The 2n3055 is a TO-3 case style.
You can use an insulator pad if you want but I just used a small amount of old thermal grease I had lying around. To mount the transistor to the heatsink I just used some spare screws and nuts I had in the garage.
Make sure that the transistor pins do not physically touch the heatsink and it is screwed in securely.
Q: Can I use a different transistor?
A: Yes you can, as long as the transistor has similar specs to the 2n3055 or better.
Q: Can I use a PNP transistor?
A: Yes, but you will have to reverse the collector and emitter connections in order for it to work. It will also need to have similar or higher specs as the 2n3055 transistor.
Q: Is the heatsink really needed?
A: Yes, if you are wanting to use this circuit for more than a few seconds the heatsink is vital as the transistor will get hot fast.
Q: Can I use a MOSFET?
A: No, a MOSFET will not work for this circuit.