Tesla coils differ in the type of switch used, the physical size of the components and the input voltage. Automotive ignition coils typically have a twelve volt input and are switched by a distributor, with moving contacts. They provide an output of 15-20,000 volts. Television fly-back transformers produce lower outputs but usually have 120 volt inputs and are switched by transistors or, in very old sets, vacuum tubes. The classic Tesla coil is switched by a spark gap. In this case, the primary circuit is known as a tank circuit. In its simplest form, the spark gap switch has two conductors separated by an air gap. When the electric field stored in the capacitor reaches a level sufficient to ionize the air within the gap a highly conductive plasma is formed, effectively closing the switch. Spark gap switched coils operate with inputs of about 5-20,000 volts and produce outputs of 100,000 to several million volts. For the spark gap to be effective, it must be able to open rapidly after the primary oscillation has damped out, in order that the capacitor may recharge. This is achieved by several methods, all of which amount to ways of cooling and dissipating the hot plasma formed during conduction. The simple gap can switch a few hundred watts of input power. Forced air cooling of the gap and, or using a number of gaps in series can increase power handling to several thousand watts. Higher power levels usually require a rotary gap, which mechanically moves gap electrodes rapidly into and out of conduction range. I should note here that even at input power levels of a thousand watts, the instantaneous power levels during gap firing can reach a million watts or more.