English: A network analyzer display showing the impedance (vertical axis) of the secondary winding of a Tesla coil as a function of frequency (horizontal axis). The horizontal axis goes from 300 kHz to 2 MHz and the vertical axis from 5000 to 500 ohms. Both axes are exponential. It shows the secondary acts as a transmission line which resonates at multiple frequencies. The oscillating radio frequency current induced in the secondary by the primary travels up the narrow coil and reflects from the top 'toroid' electrode and travels back down the coil. At frequencies at which the coil is a multiple of a half wavelength, the direct and reflected waves interfere to form standing waves, causing resonance. Each of the double peaks numbered 1 through 10 is a resonant frequency of the coil. The lowest frequency, around 370 kHz, is the fundamental frequency of the coil, at which the coil is a quarter wavelength long. The others, at about 740, 1100, 1200, 1400, 1700, 2200, 2500, and 2700 kHz are overtone resonances. Tesla coils normally oscillate at their fundamental frequency if the primary is properly tuned. Oscillations at overtone frequencies cause maxima of voltage (antinodes) along the secondary winding, which can cause arcing which can damage the delicate wire.