A Small High Frequency Oudin Coil
The high frequency transformer here described is of the Oudin type. All
that is necessary to use in conjunction with it are a small wireless
transformer or spark coil, a high potential condenser, and spark gap. The
discharge obtained when the coil is excited by a 1/4 K.W. transformer
will be from nine to twelve inches in length, while the spray discharges
will reach out to still greater lengths. The results obtained with this
small coil will thoroly repay the builder for the few hours spent in the
construction of this simple, yet intensely interesting piece of
The secondary of the transformer is shown in Fig. 1. The cardboard tube,
upon which the wire is wound, might well be a round "Quaker Oats" box cut
to length. It should be well shellacked inside and out, so as to keep out
the moisture. Two wooden ends should be turned or sawed out, and for the
sake of appearance should be nicely stained and varnished.
A discarded ball from a brass bed will be found to suit very well for a
discharger. The wire for the secondary should be numbers 28 to 30 B.
& S. D.C.C. or S.S.C. wire.
The winding must be done on a lathe or else on a simple winding rig, as
it cannot be evenly wound by hand. The turns are separated with a thread
wound on with the wire. After the wire and thread are wound, the whole
should be given two or three liberal coats of shellac (or better,
"armalac"). The upper end of the winding should be fastened to the brass
discharge ball. This finishes the secondary coil.
The primary is very easily constructed. It is made of one-inch copper or
brass ribbon. The inside turn is taped to a cardboard ring, about 5 in.
in diameter, and one inch high.
This is shown in Fig. 2. The ribbon should be wound tightly, the turns
separated by thin corrugated paper, such as used in packing boxes. The
strip of paper should be about 5/8 inch wide, so as to allow sufficient
surface of the ribbon exposed to make a variable contact with the helix
clip. After nine or ten turns have been wound, the spiral should be
tightened and taped on the outside. This makes a quite firm winding. The
inside turn is connected to the lowest turn of the secondary, see diagram
Fig. 3. Also a wire is led from this connection to one binding post.
The clip is connected to the other post by a piece of high tension cable.
The primary rests on wood supports, which may be of any convenient size.
The base should be of 3/4 in. wood, and 11 inches square. The edges
should be beveled. Porcelain knobs are glued under the corners by means
of wooden dowels inserted into the base, as shown at Fig. 4.
The complete hoop-up is shown at Fig. 3. Fig. 4 shows the transformer
assembled. In operation, the clip should be moved from turn to turn,
until the best discharge is obtained. It might be well to say that the
current from the secondary is of such high frequency that it is entirely
harmless when taken thru the body, providing the spark is allowed to jump
to a piece of metal held in the hand, and not to the skin direct.
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