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.
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.