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text of july 1964 popular electronics article on tesla coils (fwd)



---------- Forwarded message ----------
Date: Thu, 27 Feb 97 07:13:56 UT
From: William Noble <William_B_Noble-at-msn-dot-com>
To: tesla-at-pupman-dot-com
Subject: text of july 1964 popular electronics article on tesla coils

I have scanned the article in this magazine called "big TC" - there is another 
called "little tc" that I didn't bother with (uses a tube and a small coil) - 
I have tiff pictures of the pages and of the cover, and a word for windows 97 
OCR'ed version with all the graphics in it. I'm posting just the text because 
I don't know if the list server can handle attachments, and anyway, I don't 
want to upload 130K for the article unless there is interest.  I have tried 
building this coil and as far as I can tell it doesn't work - see a posting 
called "first coil experiments" for details on this.

************the article, ASCII only******************

A quarter of a million volts ? All it
takes Is a transformer, a capacitor,
a spark gap, and Tesla's  famous coil
By CHARLES CARING LLA, W6NJV
TESLA COILS have fascinated experimenters ever since the early 1900's when 
Nikola Tesla first experimented with giant coils that produced lightning-like 
discharges which would span his laboratory - the work of millions of volts of 
electricity. The Tesla coil described here is smaller than some of Tesla's 
designs, but it's capable of putting out almost a quarter of a million volts! 
Brilliant corona discharges as long as a foot or more provide a spectacular 
display of its intense electrical field, and neon and fluorescent lamps can be 
excited as far as five feet away.
Intended both as a dynamic demonstrator of electrical principles and as a 
crowd-attracting science fair project, "Big TC"" can be put together for about 
$30. However, if a used transformer from a neon sign shop can be secured 
reasonably, the cost will be even less.
#......................................................
 WARNING: The voltages used in . this project are highly dangerous.
·. Inexperienced persons should seek . aid from an instructor or other . 
expert before building it.
#......................................................

Mount L1-L2 in center of base, T1 and C1 at edges. A bigger base and greater 
component spacing will permit greater voltage output with less arcing.
As shown in the schematic diagram above, T1 steps the household line voltage 
up to 12,000 volts. The transformer is the type commonly used to operate neon 
signs. A high-voltage glass-plate capacitor, C1, is connected directly across 
the high-voltage secondary winding of T1. The capacitor serves as an energy 
storage device, charging up to T1'8 secondary voltage and then discharging in 
response to the 60-cycle a.c. voltage.
Discharging of C1 is through the spark gap into coil Ll. Each time the spark 
gap "fires," a high current flows through L1. The larger capacitor C1 is made, 
the larger will be the current through L1. Discharges across the spark gap 
produce extremely jagged pulses of power which are very rich in r.f. 
harmonics. The energy-due to the values of the components used-is greatest in 
roughly the 100-kc. region.
Windings L1 and LS form a air-core step-up transformer, with L1 the primary 
and . ,2 the high-voltage secondary. The voltage at L2 will be 75,000 to 
250,000 volts depending on the size of C1.
Design and Layout. The prototype of "Big TC" was built on a plywood base 
measuring 3/4" x 22" x 22", although a larger base would be desirable for 
highvoltage units to prevent arcing between LS and T1 and C1. Mount L2 in the 
center of the base and T1 and C1 as close to the edges as possible; if you 
plan to operate the unit at voltages exceeding 100,000 volts, make the base 3' 
x 3' for even greater separation between components.
Power transformer T1 is the only
Spark gap generates r.f. energy to excite coil. It consists of two copper rods 
mounted on standoffs.
Big TC" is relatively simple, a' schematic reveals. Capacitors connected in 
parallel with C1 can bs used to increase output voltage

The various dimensions of the prototype coil are indicated in the drawing; 
none is particularly critical. Note that space has been left at each end of 
coil, and that stand-off insulators are used to bring out the ends. Nylon 
screws or glue must be used to fasten top end cover to avoid arcing. After 
winding coil, cover with many coats of acrylic plastic spray. Spray form first 
if cardboard is used. Details on spark gap are shown at right.
Constructed for classroom demonstration, the author's unit was mounted on 
mahogany veneer plywood which was sanded and covered with five coats of 
plastic varnish. Other finishing touches were wood tape veneer around the 
edges, and nylon casters to make the unit easy to move.

Form for L1 with center cut out to take base of L2 is shown above. Polystyrene 
was used, but wood or cardboard can be substituted. The critical dimension is 
the outside diameter; less than 9~ will result in arcing between coils L1 and 
L2.

Leads are soldered directly to capacitor plates. Note use of stand-off 
insulators.


Glue metal plates to glass, leaving a generous margin of glass on all sides 
(see text). Epoxy glue, contact cement, or any other glue which will form a 
tight bond can be used. The wood frame protects the glass and makes mounting 
it possible.
high-cost component. A neon-sign unit rated at 12,000 volts a.c. at 30 ma., it 
sells for about $40 new, but used transformers are constantly being salvaged 
by sign shops, and can be picked up for $10 to $20. It is also possible to 
find neon signs in junk yards, in which case you can probably buy the 
transformer for practically nothing. The author used a GE unit, No. 
51G473,-known technically as a "luminous tube transformer." Measuring 9l/2'' x 
6" x 4", it has 2" feedthrough insulators at either end connecting to the 
high-voltage winding.
Primary coil L1 and all connecting leads must be made with high-voltage wire 
preferably supported away from the base on 1" ceramic standoff insulators. 
Test prod wire such as Belden Type 8898 is ideal-it has flexible rubber 
insulation with a puncture voltage rating of 29,000 volts.
Winding the Coil. For the big coil ( L2 ) a phenolic coil form* measuring 
43/4" in outside diameter and 38" in length was used. Alternately, cardboard, 
wood or other insulating materials can be substituted. You can improve these 
latter types of coil forms by spraying on at least six coats of acrylic 
plastic spray before winding the wire on them.
The winding itself is done with No. 26 Formvar-insulated wire-two 1-lb. spools 
( splice them together and keep the solder joint as small as possible ) will 
give you a 2000-turn, tightly spaced coil covering 34 1/2" of the coil form. 
There should be extra space between the ends of the winding and the ends of 
the form-see the drawing on page 31.
The lower end of the coil is terminated at a 1" feed through insulator 
installed in the side of the form, the top end of the coil at a 41/2" 
feedthrough mounted to the top end of the form. Make the end covers of wood or 
phenolic discs cut to the inside diameter of the coil form and mount them in 
place with nylon screws (metal screws at the top end would produce corona 
discharges which could burn the coil form). Alternatively, the top coil cover 
can be cemented in place with epoxy cement if a sturdy coil form is used. The 
coil is attached to the base with a 3/8" bolt.
Winding the coil is not nearly as difficult as it appears-the author completed 
the task in about two hours. Spray the entire winding with acrylic plastic for 
added insulation, moisture protection, and to keep the windings in place. You 
can't overdo this step-the author used the contents of an entire aerosol spray 
can on the prototype, applying one thin layer at a time and letting it dry 
before adding another.
Building the Primary. As shown on page 31, the form for L1 was made with 
polystyrene rods and sheeting. While the plastic has excellent insulating 
qualities and looks attractive, wood or even cardboard can be substituted. If 
plastic is used, it can be strongly "welded" together with acetone. Regardless 
of the material used, the form should have an outside diameter of at least 9" 
to avoid arc-over between L1 and L2. The coil itself ( L1 ) consists of 20 
turns of heavy test prod wire.
Spark Gap. The spark gap is simply two ordinary binding posts mounted on 
stand-off insulators. In turn, these are mounted on a phenolic base measuring 
3/8" x 21/4" x 6". The electrodes are brass and copper rods with a gap on the 
order of 1" between them. This distance will vary slightly, depending on the 
size of capacitor C1.
Fabricating the Capacitor. The capacitor consists of two 14" x 14" sheets of 
tin cemented to a 18l/2''-square piece of window glass. Although aluminum foil 
can be used for the capacitor plates, tin was obtained from a sheet metal shop 
for this purpose so that connecting leads could be soldered directly to it. If 
you use aluminum foil, a fairly good connection can be had by making leads of 
l/2"-wide aluminum foil strips and taping them down to the electrodes.
Glass is an excellent dielectric material for this application since it has an 
extremely high puncture voltage and a high dielectric constant. As you will 
note in the drawing on page 32, a border of glass is left around the capacitor 
plates-this should be at least 1 l/2'' wide. The calculated capacity of C1 is 
approximately 0.0027 pf.
Testing and Operation. Caution! Adjustments to the Tesla coil, and 
specifically to the spark gap, should be made only when the unit is o.ff. 
Although the output voltage of the Tesla coil may be on the order of 150,000 
volts, the current capacity is only hundreds of microamps. This current can 
inflict a nasty shock and r.f. burns, however.
Use EXTREME CAUTION around the neon sign transformer. It delivers 12,000 volts 
at 30 ma., and this voltage could be lethal under certain conditions. Again, 
be sure the plug is out when you make adjustments.
To adjust the spark gap, first open it to about 11/2"; it will not fire at 
this point. Gradually move the electrodes together-unplugging the unit each 
time you adjust the gap-until the point is reached where the gap "fires."
The author's version of "Big TC" produced an output voltage of 100,000 volts 
with the 0.0 ~27-~uf. capacitor described. To increase the output voltage, 
simply construct one or two more capacitors and parallel them across C1. With 
two capacitors in parallel, the prototype Tesla coil produced 150,000 volts; 
with three capacitors, 200,000 volts. However, it began to break down between 
coil L2 and capacitor C1 above the 200,000-volt region. As mentioned earlier, 
greater output voltage can be obtained by making the base larger and 
increasing the spacing between components to eliminate arcing.
The output of your Tesla coil can be estimated by drawing an arc to a metallic 
object attached to a long wooden handle. Slowly increase the distance between 
the object and the discharge terminal until the arcing stops: a 6" arc 
represents 100,000 volts, a 14" arc about 200,000 volts, and a 21" arc some 
300,000 volts. More amazing than figures, however, are the brilliant, 
spectacular phenomena exhibited bv high voltage. high-frequency electricity.

*Tubing can be found in metropolitan areas at surplus houses and 
establishments which sell plastics (sheets. rods. etc. . Clear acrylic tubing 
(48" long, 414" O.D.) can be ordered from Industrial Plastics Supply Co., 324 
Canal St.. New York, N. Y. 10013. for $13.85 including shipping charges and 
postage; address your order to the attention of Mr. Charles Roth.