Construction of a Small Tesla Coil

To those experimenters having only a small spark coil (mine is a 1/4-inch Ford ignition coil) and wishing to produce High Frequency currents, I describe here a coil I made which proved very satisfactory. I get a H.F. spark nearly 3/4-inch long. When the electrodes are wide apart the brush discharge (in the dark) is very marked and beautiful. A 6 1/2 inch Geissler tube begins to glim when placed 3 inches away from coil, if placed between electrodes, and 1/2 inch away from them, the tube lights up brightly. Even a small X-ray tube lights up.

The coil was constructed with use of only a few tools. The wooden parts are 3/16 inch cigar-box wood, except the base, which is 1/2-inch common wood. For the primary I cut out 2 rings 2 1/2 inch outside diameter 1 1/4 inch inside diam. These are connected by 6 crossbars 1 1/8 inch long and about 3/8 inch wide. After the drum is fastened together with wooden "nails" (pegs) (don't use metal nails or brass screws) and boiled in parafine, it is wound with 5 layers of brass picture wire (or any stranded wire baring iron) with equal distances between layers. The whole is glued between 2 blocks in the center of the baseboard. The secondary is a cardboard tube 1 inch wide 4 1/2 inches long. The tube ends are closed with wooden discs. After boiling in wax it is wound (close and even) with No. 34 or 36 S. S. C. wire to within 1/4-inch of the end. The supports for secondary are an old fountain pen holder cut in two, forced and glued into two pieces of wood. 1 1/4 inch from bottom a hole goes right through and brass rods hold the secondary, exactly in the center of the primary. A thick brass wire with a binding screw on top is fixed into each support and the ends of secondary wires are connected to these. The condenser consists of 6 plates (3 couples) the foils are 4 1/4 x 2 1/4 inch. The spark gap consists of 2 iron screws 1/8 inch thick with nuts 1 1/2 – 2 inches long, screwed in. 2 supports (vulcanite pipe mouthpieces) The nuts serve to connect wires. The gap is about 1/16 inch. The connections are from secondary A of spark-coil to condenser and spark-gap; from Sec. B of spark-coil to the other side of condenser and primary A of Tesla coil. Primary B. of T. C. is connected to other side of spark-gap. The whole apparatus is well shellaced to insure good insulation, which is very important.