|Jochen's High Voltage Page|
Sparks, in contrast to arcs, are defined as single, very short electric discharges across an air gap. They are accompanied by a more or less impressive bang and a flash of light. After the spark, the voltage source must first be recharged before the next event can take place.
Physically, the bang and flash of light are produced when a tube of very hot, ionized air (a so-called plasma) forms and expands rapidly. The ionization is first caused by the acceleration of single random charged particles in a strong electric field, resulting in new ionization when the particles hit air molecules. Once a conductive channel has formed, it is strongly heated by the resulting short-circuit current from the voltage source, which can be very high if a capacitor is involved.
This is a time exposure of sparks jumping around the edge of a porcellain plate (approx. 25cm diameter). The electrodes are centered on both sides of the plate. As a voltage source serve two car ignition coils in series, delivering an estimated maximum of 60kV in the form of a high energy short pulse. The pulse is generated by the discharge of a large electrolytic cap through the primary windings of the coils. A discharge path along the surface of an insulator, as is the case here, can be much longer than the maximum air gap at the same voltage, which in this example would be around 10cm.
This picture was taken in a darkened room by keeping the shutter of the camera open for about 7 discharges. Before the shutter was closed again, the scene was slightly illuminated using a photo flash, to give some impression of the dimensions.Larger version
This is again a time exposure, of about 10-20 discharges of the super-cascade. The gap between the sperical electrodes is about 20cm. A resistor (visible at the right hand edge) limits the current to protect the cascades. This also results in a much reduced brightness and loudness, the discharge is longer in time.
The voltage in this experiment is about 120kV. To achieve the maximum spark length, the size and shape of the electrodes has to be optimized (by trial and error). The general rule, that the smaller the electrodes, the longer the spark, has its limit when the corona current causes such a large voltage drop that the formation of a spark is no longer possible. Therefore, the optimal electrode size will be somewhere between the extreme cases of needle points and parallel plates. In my case, it proved useful to have a large electrode (metal hot-water bottle) to suppress corona and a somewhat elevated, sperical electrode (2cm diameter) from which the sparks originate.Larger version
|Voltage sources used for spark experiments tend use extremely high voltages - keep a respectful distance. Also, capacitors which store a lot of energy make sparks brighter and louder, but are also quite deadly. Make sure they are fully discharged before touching them. Bleeder resistors are recommended.|
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