Making the Coil and components
Winding the Secondary coil
The secondary coil consists of 1000T of enamelled copper wire wound on a
length of heavy polythene water pipe about 125mm in diameter.
(The wire thickness is about 0.52mm including the enamel coating.)
The length of polythene tube was held in the three-jaw chuck at one end, with the other end jammed over a rotating centre. The outer surface of the tube was first turned accurately cylindrical, and then a single shallow V thread of 0.6mm pitch cut over a length of about 620mm. The purpose of this was to locate the turns more rigidly onto the former, and to give a small amount of extra spacing to reduce self-capacitance and improve insulation between turns.
The bottom (chuck) end of the winding was secured by soldering it to a short length of galvanised steel wire, which had been pushed into the polythene former while heating it with the soldering iron. The copper wire was fed onto the rotating former by means of a small piece of hardwood with a slot cut in it, held in the toolholder. The same feed (0.6mm / turn) was used. Coil winding took about 3 minutes. This had a certain inevitability about it, as the lathe was run at 330 rpm. The wire was fed and tensioned manually from a large reel held by means of a round metal bar.
After several days drying in an airing cupboard, the coil was given three coats of Hardglaze Polyurethane varnish.
In use, flashover was found to occur through the hollow centre of the former. To prevent this, a thick disc of polythene was turned to be a press fit inside the tubular former, and the blocked tube filled with candle wax to a depth of about 150mm. (The molten wax was poured in several batches to minimise shrinkage problems as it solidified.)
Aluminium Sphere Terminal
The spherical terminal was formed by glueing together two spun aluminium hemispheres using a 9mm thick plywood ring and epoxy adhesive.
The inside lip of each hemisphere was scored and roughened before the adhesive was applied. Three dressmaker's pins were applied centrally around the outer edge of the plywood ring so that it could only be inserted to half of its thickness. When the adhesive had set, the pins were removed, and the second hemisphere was glued. A 10mm dia. GRP dowel was glued through a hole in the centre of the bottom hemisphere, and through the centre of a plywood disc. The disc was also glued to the inside of the bottom hemisphere. The purpose of this arrangement was to support the sphere centrally at the top of the secondary coil, and to prevent it being accidentally dislodged and damaged.
(Hemispheres still available - please email.)
Spark Gap
The spark gap consists of eight 120mm lengths of 22mm dia. copper tube spaced 0.5mm and mounted in an open varnished MDF frame.
This form of construction has a number of advantages. It is simple and easy to make, very easy to clean and service, has excellent natural cooling by convection, adapts easily to air blast cooling/quenching, and is easy to adjust by placing small metallic objects between the tubes to short out individual gaps. The tubes are canted to increase the gaps between the ends, to confine the arc to the central region. The tube sections can be held in place with an epoxy putty such as Milliput.
Capacitor
The capacitor is made in the form of a slotted rack with copper busbars running down each side. It is designed to accept "trays" made from varnished MDF with one or two series strings of 0.047uF 1500V capacitors (Shown above.) The individual capacitors are pressed into beads of silicone sealant. The end connectors of the trays locate on 4mm brass studs on the busbars, and are secured with wingnuts. This design enables the value of the primary capacitor to be varied incrementally, and the easy replacement of defective strings.
Primary Coil
This consists of 10 turns of 10mm copper tube wound in a conical spiral on eight wedge-shaped supports cut from MDF. The 10mm semicircular notches were milled on a 20mm pitch using a 10mm endmill. The pattern was displaced 2.5mm on each wedge, to accommodate the spiral. (The use of the milling machine for this was probably overkill, but it was there, and made the job easy and accurate.) The wedges were dried carefully for several days, and then given several coats of polyurethane varnish, before being glued to the circular top. (Experiment has shown that MDF treated in this way has a surprisingly high breakdown voltage. It is critical that it is dried thoroughly before sealing.) The base unit was painted gloss black before the primary coil was fitted. Note that the top 15mm of the wedges was masked off during this process. The black paint probably contains lampblack as a pigment, and is not an insulator! Note also the polythene feedthrough insulator at the centre of the primary for this reason. The primary spiral was fitted to the notches by hand (rather like wrestling with a snake) and then glued and clamped into each notch with epoxy adhesive. A brass ferrule was soldered into the outer end of the tube to strengthen it, and this was screwed to the end of the adjacent MDF support wedge. (The hole for this screw had previously been lined with GRP dowel as an insulator.) The bend at the centre of the primary is a standard 10mm plumbing fitting.
(Note brass spring contact for secondary coil earth on central boss.)
Brass Terminals (Earthing)
It seems impossible to obtain large, decently-made brass terminals nowadays. Have brass bar, have lathe....
SAFETY: As has been stated in most other references, the construction and use of this sort of apparatus is extremely hazardous. If you have the slightest doubt as to your ability to do this safely, do not attempt it.
The information given above should not be taken as a recommendation as to safe or best practice. It is the record of one person's attempt to make a coil, and is there merely for the possible edification and probable amusement of the reader.
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