just a quick question, I saw a video on Tesla Coils and found out that the Spark gap transmitter is quite old and yet the design is still being used. What purpose does it serve? What alternative is there to the spark gap?
The spark gap allows the build up of Voltage up to a certain point, until the arc will jump the gap resulting to the collapse of the magnetic field of the coil. The process repeats generating the high voltages. If you know the gap distance, then you can calculate the output voltage If you charge a capacitor and set a given gap, the capacitor will charge up, exceed the voltage for the gap, and a spark will fire across the gap. the capacitor will then charge again, and fire again...and again and again... You can control the resultant high-voltage frequency by increasing or decreasing the spark gap. the output voltage and frequency in the secondary coil reflects that frequency and voltage.
The other thing the arc does is create a fast rise-time rich in high-frequency harmonics. This helps the air-core inductors used for the coils to operate more efficiently (if they had an iron core they'd work better at 60 Hz, right?).
Just as the rapidly collapsing field in an ignition coil helps to boost the secondary voltage, so does the rapidly rising field in a Tesla Coil primary.
Oh, it also ensures you will blow out radio reception for a pretty big radius: A good spark-gap will make harmonics of the 60 Hz primary power basically from DC to light.
I built one of these in about 1962, and won a blue ribbon for a 4H electric project at the state fair. Used a neon light transformer as the power source, aluminum foil glued to window glass for a capacitor (3 sheets, each about 10 by 12 inches) a nine turn primary made from RG-58 with the braid stripped off, and a 2000 turn secondary. The spark gap was two pointy copper wires mounted in binding posts. I could draw an arc from the secondary about 5-6 inches, and light a flourescent tube from about 3 feet away. And yes, it interfered with radio and TV in our house.