the secondary coil Assuming no energy losses, . Substituting into this equation and simplifying, the peak secondary voltage is The second formula above is derived from the first using the resonance condition . Since the capacitance of the secondary coil is very small compared to the primary capacitor, the primary voltage is stepped up to a high value. The above peak voltage is only achieved in coils in which air discharges do not occur; in coils which produce sparks, like entertainment coils, the peak voltage on the terminal is limited to the voltage at which the air breaks down and becomes conductive. As the output voltage increases during each voltage pulse, it reaches the point where the air next to the high-voltage terminal ionizes and corona, brush discharges, and streamer arcs break out from the terminal. This happens when the electric field strength exceeds the dielectric strength of the air, about 30 kV per centimeter. Since the electric field is greatest at sharp points and edges, air discharges start at these points on the high-voltage terminal. The voltage on the high-voltage terminal cannot increase above the air breakdown voltage, because additional electric charge pumped into the terminal from the secondary winding just escapes into the air. The output voltage of open-air Tesla coils is limited to a few million volts by air breakdown, but higher voltages can be achieved by coils immersed in pressurized tanks of insulating oil. Top load or "toroid" electrode Most Tesla coil designs have a smooth spherical- or toroidal-shaped metal electrode on the high-voltage terminal. The electrode serves as one plate of a capacitor, with the Earth as the other plate, forming the tuned circuit with the secondary winding. Although the "toroid" increases the secondary capacitance, which tends to reduce the peak voltage, its main effect is that its large-diameter curved surface reduces the potential gradient (electric field) at the high-voltage terminal; it functions similarly to a corona ring, increasing the voltage threshold at which air discharges such as corona and brush discharges occur. Suppressing premature air breakdown and energy loss allows the voltage to build to higher values on the peaks of the waveform, creating longer, more spectacular streamers when air discharges finally occur. If the top electrode is large and smooth enough, the electric field at its surface may never get high enough even at the peak voltage to cause air breakdown, and air