Air-Insulated Tesla Coil
By Leonard R. Crow
Experimenters who have wireless sending stations have all the apparatus
necessary for the operation of an air-insulated Tesla coil.
The coil, here described is suitable for use with spark coils giving from
a 2-inch to 6-inch spark or with a 1/4-kw. transformer.
The coil described in this article will give a 7 1/2-inch spark when used
in connection with a 1/4-kw. closed core transformer (magnetic leakage
type ), glass plate condenser and a rotary spark gap. When a stationary
gap is used the sparking distance is about 5 inches.
The illustration gives the necessary dimensions and shows part of the
primary and secondard tubes cut away to bring out various points in the
The primary consists of 21 turns of No. 14 B. & S. gauge
rubber-insulated wire wound on a paper or fiber tube 6 inches outside
diameter. This tube is easily made from a pasteboard box, which may be
procured at any grocery store.
This is also true of the secondary tube. The length of the primary tube
is not shown in the illustration because there is a great variation in
the thickness of the insulation of the different kinds of wire. There
should be just enough space between the end pieces to accommodate 21
turns of the wire used. If No. 14 D. R. C. wire, which is almost 1/4 inch
in diameter, similar to that for electric light wiring, is used the space
on the tube should be about 4 inches for 21 turns. This wire can usually
be obtained, although stranded wire is preferable.
The secondary coil consists of one layer of No. 32 B. & S. gauge
enameled or D. S. C. wire (cotton-covered may be used) wound on a tube 3
inches outside diameter. The winding should be 11 1/2 inches long. The
base and woodwork may be of any size the builder desires. The posts,
however, must not be lower than indicated or the spark will jump into the
The framework may be made of most any hardwood, or it can be made of
poplar, but hard pine should not be used. The coil should be put together
entirely with wooden dowels, because if nails or screws are used the
sparks will jump into them.
The posts may be forced into half-inch or larger holes bored in the base
to receive them and should be held with glue.
The tubes may be fastened to the end pieces with glue, but it is
preferable to fasten the larger tube into its end pieces by cutting a
hole in each piece just large enough to allow the tube to slip into each
end and fit snugly. See illustration showing the construction of these
Holes may be bored in the base and in the end pieces to fasten them
together with wooden dowels and glue. This is accomplished as easily as
by using screws.
The next thing is to fasten the secondary into place. The best way to do
this is to insert the two dowels projecting from the end of the secondary
into the two holes bored in the upright posts to receive them and gently
drive both posts into the holes made in the base for them. This completes
the construction of the coil. The woodwork may be finished with any
substance that is a good insulator.
The size of the condenser will depend upon the spark coil or transformer
used and will have to be determined by the experimenter.
For spark coils of about 2 inches a half-gallon Leyden jar may be used,
but with larger coils the number of the jars should be increased until
the correct capacity is obtained. This would also be true of plate
Very beautiful experiments may be performed with this coil in a dark
room. The spark may be allowed to jump to a piece of metal held in the
hand, but no sensation of pain will be experienced. If the spark jumps
directly into the body only a slight burning sensation will be felt.
Lubricating oil may be placed on the end of the finger and lighted by
means of the spark without burning the finger.
A very surprising and beautiful, as well as weird, effect may be produced
by connecting small cotton-covered magnet wire, No. 32 or smaller, to
each terminal of the secondary and leading them off to the side of the
room, keeping the wires about a foot or two feet apart. It is, of course,
understood that all Tesla experiments should be performed in absolute
darkness, as the desired results may be obtained only in this way.
Another experiment is to ground one wire of the Tesla secondary and
attach a piece of wire to the other secondary. This will cause an
enormous brush effect on the free wire.
Scores of other equally interesting experiments may be carried on, but
these the experimenter may easily find out for himself.
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