Re: Surface Sparks = Wow!!

From: 	Robert Michaels[SMTP:robert.michaels-at-online.sme-dot-org]
Sent: 	Friday, August 01, 1997 3:11 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Surface Sparks = Wow!!

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From: robert.michaels-at-online.sme-dot-org (Robert Michaels) 
Subject: Surface Sparks = Wow!! 
Date: Fri, 01 Aug 1997 20:11:15 GMT 
Message-Id: <870466275-at-online.sme-dot-org> 
Organization: Society of Manufacturing Engineers 
To: tesla-at-puman-dot-com 
        Dear People: 
        This post is inspired by the several recent ones on the tendancy 
        of spark discharges to follow a surface and evince a longer 
        sparking distance than would be obtained from the same voltage 
        discharging in free air: 
        This surface-enhanced sparking can be exploited to make a very 
        entertaining "appliance" for use with small to medium sized 
        Tesla coils: 
        1) -  Obtain a sheet of suitable insulating material.   
                Most any will do, but let's say acrylic plastic  
                ("Lucite"; "Plexiglas"; "Perspex"; et. al.) 
        2) - The effect is enhanced if the acrylic is black, but the 
             backside may be painted if it is colorless. 
        3) - The size of the sheet is determined by the size of the 
             coil, but let's say 4-ft. x 4-ft. for the sake of dis- 
        4) - The esthetics seem enhanced if the square sheet is cut 
             into a circle of the same diameter, but this is immaterial. 
        5) - Drill a small hole in the exact center of the acrylic sheet 
        6) - Cut a circle of aluminum, say 1-ft. in diameter, more 
             or less.  Either light sheetmetal or heavy foil may be 
        7) - Paste the aluminum circle in the  =exact=  center of the 
             acrylic sheet.  You're on your own as to how to "paste". 
             I use shellac because it is very traditional. 
                        I also use gutta percha (in other applica- 
                        tions) for the same reason -- but I digress. 
        8) - Make electrical connection to the aluminum circle thru 
             the hole previously drilled in the acrylic plastic. 
             An electical lead connected here will eventually connect 
             to the business end of the Tesla coil. 
        9) - Form a length of copper tubing (or heavy copper wire) 
             into a 4-ft. dia. circle. 
        10) - Precisely center this around the aluminum circle and 
              adhere the tubing (or wire) to the acrylic plastic. 
        11) - Make electrical connection to the copper tubing.  This 
              goes to the rf-ground of the Tesla coil. 
        12) - Turn on the Tesla coil and let it rip. 
        A hissing, snapping circle of sparks jumps from the perimeter 
        of the aluminum disk to the copper tubing -- usually from 
        dozens and dozens of places at once, continuously dying out 
        and reforming in another spot, sometimes rotating around the 
        disk perhaps first clockwise, and then counterclockwise. 
                                - - - - - - - -  
        A four ft. piece of acrylic is a convenient size with which 
        to experiment, but many a small Tesla coil will support 8-ft., 
        10-ft., and even 12-ft. diameters. 
        The hardest part in all of this is getting the perimeter of 
        the aluminum disk everwhere equidistant from the copper 
        tubing.  If the two are not precisely on the same center,  
        the sparks will tend to concentrate at the one spot where 
        the two are closest.  They will not rotate around the disk 
        nor uniformly encircle it. 
        In such event, the offending point on the aluminum disk 
        may be trimmed off (gradually!) until the disk and the copper 
        are equidistant.  Use a utility knife for this if the aluminum 
        is foil, else a Dremel "Moto-Tool" or the like. 
                                 - - - - - - - 
        This general idea of a disk surrounded by an encircling ring 
        of copper comes from no less an august personage than Dr. 
        Tesla himself, but ASFAIK Dr. Tesla mounted his disk and  
        ring in free air and did not avail himself of the ability of 
        a surface to greatly enhance spark distance. 
                                           Sparking (and partying) on 
                                           in --- Detroit, USA 
                                           Robert Michaels