A Tesla Coil is made up of several components, namely:
Also, a Tesla Coil needs a very good earth connection, or else another coil (with the same specifications as the secondary coil) is used as a virtual earth by wiring it up as a centre tap.
The primary and secondary coils act as a transformer which, under the right conditions, can generate a very high (and lethal) voltage output. Unlike low-frequency transformers, a Tesla Coil has an air core and no iron is used in it construction.
Typically, the transformer is usually a Neon Tube Transformer with ratings such as 15kV, although I would recommend something more .. docile.. for the aspiring coiler.
The spark gap is one of the most crucial factors in the entire setup. There are many different types of spark gaps, and one must make sure that it is precise and quenched. Adequate quenching is achieved by using a rotary gap, by spreading out the arc over a series of static gaps or by placing a strong magnetic field between the electrodes. At higher powers (over 5 kVA), one has to use a static spark gap or a combination of rotary and static spark gaps. The ultimate spark gap is achieved through a series of trials and errors.
Leyden jars or a capacitor bank (that is, a series of capacitors lined up together) are used to generate the pulse by passing current back and forth across the primary coil.
The toroid or sphere on top is used so as to increase the surface area and reduce the likelihood of any sharp point effects. One may have to play around with these a bit so as to obtain the optimum spark.
Warning: The spark can be lethal, so watch out