To figure how large a spark gap you can accommodate, or how much power you need to make it work, divide the length of the spark gap in inches by 1. 7 and square it to determine the input power in watts. (Conversely, to find the spark gap length, multiply the square root of the power in watts by 1. 7. ) A Tesla coil that creates a spark gap of 60 inches (150 cm) (1. 5 meters) would require 1,246 watts. (A Tesla coil using a 1-kilowatt power source would generate a spark gap of almost 54 inches, or 1. 37 meters. )
Capacitance is the ability to hold an electric charge or the amount of electric charge stored for a given voltage. (A device designed to hold an electric charge is called a capacitor. ) The unit of measure for capacitance is the farad (abbreviated “F”). A farad is defined as 1 ampere-second (or coulomb) per volt. Commonly, capacitance is measured in smaller units, such as the microfarad (abbreviated “uF”), a millionth of a farad, or the picofarad (abbreviated pF and sometimes read as “puff”), a trillionth of a farad. Inductance, or self-inductance, is how much voltage an electric circuit carries per the amount of current in the circuit. (High-tension power lines, which carry a high voltage but a low current, have high inductance. ) The unit of measure for inductance is the henry (abbreviated “H”). A henry is defined as 1 volt-second per ampere of current. Commonly, inductance is measured in smaller units, such as the millihenry (abbreviated “mH”), a thousandth of a henry, or the microhenry (abbreviated “uH”), a millionth of a henry. Resonant frequency, or resonance frequency, is the frequency at which the resistance to transfer of energy is at a minimum. (For a Tesla coil, this is optimum operating point for transferring electrical energy between the primary and secondary coils. ) The unit of measure for the resonant frequency is the hertz (abbreviated “Hz”), defined as 1 cycle per second. More commonly, the resonant frequency is measured in kilohertz (abbreviated “kHz”), with a kilohertz being equal to 1000 hertz.
Your power source/transformer feeds power through the chokes to the primary, or tank circuit, which connects the primary capacitor, primary inductor coil and spark gap assembly. The primary inductor coil is placed adjacent to, but not wired to, the inductor coil of the secondary circuit, which is connected to the secondary capacitor. Once the secondary capacitor has built up sufficient electric charge, streamers of electricity (lightning bolts) discharge from it.
Small capacitors, and their associated bleed resistors, can be obtained from electronics supply stores, or you can scrounge for ceramic capacitors from old television sets. You can also make the capacitors out of sheets of polyethylene and aluminum foil. To maximize the power output, the primary capacitor should be able to reach its full capacitance each half-cycle of the frequency of the power being supplied to it. (For a 60 Hz power supply, this means 120 times each second. )
The length of the cord determines the inductance of the primary coil. The primary coil should have a low inductance, so you’ll use comparatively few turns in making it. You can use non-continuous sections of wire for the primary coil, so that you can hook sections together as necessary to adjust the inductance on the fly.
If you lack the materials to make the secondary coil tall enough, you can compensate by building a strike rail (essentially a lightning rod) to protect the primary circuit, but this will mean that most of the Tesla coil’s discharges will hit the strike rail and not dance in the air.
Your secondary circuit should be grounded separately from the grounding for your household circuits supplying power to the transformer to prevent a stream of electric current from traveling from the Tesla coil to the ground for your household circuits and possibly frying anything plugged into those outlets. Driving a metal spike into the ground is a good way to do this.
If the primary coil is of sufficiently large diameter, the secondary coil can be set inside it.
