The Spark gap....  the noisy part of the coil. This device is the switch that jumps on and off when the voltage present in the capacitor is at the value determined by the width of the spark gap distance. In other words...  small gap setting = low voltage required to jump the gap...   wide gap setting = higher voltages needed to jump the gap.  To wide of a gap and its goodbye capacitor. The standard for the gap space seems to be about 1/3rd of an inch for small to medium powered coils.  If you open the gap much further than this standard, you increase the chances of blowing up the capacitor(s).

        Spark gaps can be as crude as 2 nails stuck in wood ( single gap set) , to as complex as Synchronous Rotory Gaps. The simplest design consists of 2 electrodes spaced apart and normally having at least one of the electrodes adjustable ( moveable inwards - outwards) to enable you to fine tune the gap width.

        The main problems encountered with spark gaps are that they produce immence amounts of heat, they erode quickly, they produce ultra-violet light that can cause retinal damage ( just like welding machines).  Iron based metals dont last long in this enviroment, they pit, melt, and vaporize in a matter of minutes. The choice of material for such a gap is Tungsten rods. They handle the heat better, they are less prone to vaporize, and last longer than most any other material.

        A popular type of spark gap is the series gap, this consists of several 1" copper pipes laying next to each other. The gap distances between the pipes are at a much smaller distance than found in the single gap set. But, the total distance of all the gaps normally equals the distance found in the single gap set. There are several variations of the series gap set. The simplest being several pipes paralell to each other on a flat base. The next type is the RQ gap, the pipes are set inside a piece of large diameter PVC pipe along the interior wall. Then there is the AMAZING linear SD gap  ;) ...

        This gap ( the SD gap) is a spin off of the RQ gap and the flat base gap. It allows the gap to be adjustable, easily cleaned, and built with basic tools and a accurate measuring device.

        The more complex gap sets are the rotory gaps. These incorporate an electric motor spinning a disc with several electrodes mounted in the disc. As the Disc spins, the electrodes move past a set of stationary electrodes, completeting the circuit. Care must be taken in choosing the proper material for the disc due to the high forces involved with the stresses the electrodes place on the material. Besides dealing with a chunk of metal in a spinning disc, the disc electrodes also are subjected to a goodly amount of heat. Therefore the disc material must be strong, have a high insulative property, and be very heat resistant. The usual choice for such a material is G-10 phenolic/resin stock.
 
 

              Rotory Gap set with Lexan Disc                            Single Air blown gap   Tungsten rods

             RQ Gap Set                                      Linear Gap Set with vacuum air flow            Smaller version of the SD gap
 

            3 Views of the SD Gap set
 
 

                          Synchronized Rotory gap
 
 
 
 
 
 
 
 
 
 
 
 

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