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Magnet Design for Tesla Coils




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Folks:

Just some notes on electromagnet design and references.  Building a cyclotron
is undoubtedly a very challenging project.  However, designing electromagnets
to magnetically quench a spark gap in a disruptive Tesla Coil is not nearly
as difficult.

Some folks will debate the utility of magnetic quenching applied to small
Tesla Coils, but I think it has good possibilities.  Tesla used magnetically
quenched gaps in some of his designs and those details are covered in "The
Inventions, Researches and Writings of Nikola Tesla," by T.C. Martin (1894)
and reprinted by Barnes & Noble Books. This book contains a lot of other
great information and has been mentioned before on the Tesla List.

To put things in perspective, I built a "large" (everything's relative!)
electromagnet and will relate some of the design features and problems.  This
particular magnet was not used to quench spark gaps, but the design
principles are the same.  This magnet used two coils mounted opposite each
other on a mild steel frame and this configuration is known as a "Weiss
Electromagnet' (Bleaney, et al, 1957).

Each coil consisted of 6 lbs. of #18 magnet wire, "scramble-wound" on 2 1/8"
diameter cold rolled steel (mild steel) pole pieces.  The pole piece faces
were machined at a 60 degree angle so the face was 1" across.  Magnet gap
width was adjusted with steel washers as spacers.  Power supply was either a
filament tranformer with rectifier or a Variac-rectifier combination. Magnet
coil resistance was apprx. 15 ohms at 70 degrees F (resistance obviously will
increase as coils warm up).

With the magnets' gap set at 2 cm, a 6 ampere current at approx. 100 VDC,
produced a 10K Gauss magnetic flux in the center of the gap.  Measurements
were taken with a Bell Gaussmeter (expensive item, but not if you can borrow
one!).  This is not bad and would probably be higher if the coils were not
scramble-wound.  However, at this current, the coils heat up rather quickly
and that's not good (can use larger gauge wire).  And yes, you can calculate
the magnetic flux:  that's easy for an air-core solenoid, but gets tedious
with electromagnets.  At lower currents, this particular magnet serves its
purpose of producing moderately strong fields from 1K to 5K Gauss without any
dangerous rise in operating temperature. 

So, in that regard the magnet was successful.  Again,  this is not really a
big magnet by many standards, but similar designs can be used to quench spark
gaps in disruptive Tesla Coils. An important difference in this application
is insulating the spark gap from the magnet pole pieces (whether you use a
permanent gap magnet or electromagnet).  This is not as easy as it sounds,
especially when you don't have exactly what you need and must "improvise"
(we're all kind of used to that!).

Applicable references follow and are certainly not all inclusive. Original
publication dates included;  these references are still available, except for
Bleaney (that's why it's a good idea to "cruise" used book shops). These are
useful references that cover many facets of magnet design.  Other resources
include most electrical engineering texts (especially the older ones) and
physics text books.

Underhill, C.N.  Solenoids, Electromagnets and Electromagnetic Windings.
Reprinted by Lindsey Pubs., 1921.

Lee, E.W. Magnetism - An Introductory Survey.  Dover Pubs. (and Lindsey Pubs,
I believe), 1970.

Bleaney, B.I. Electricity and Magnetism. Oxford University Presss, London,
1957.

Montgomery, T.B. Design of Magnets and Electromagnets.  Reprinted by Lindsey
Pubs., 1948.

Chabay, R. and Sherwood, B.  Electric and Magnetic Interactions.  John Wiley
& Sons, Inc., New York, 1995.

Fretter, W.B.  Introduction to Experimental Physics.  Prentice-Hall, Inc.,
New York, 1954.

Gingery, D.J. How to Build a Magneto Magnetizer.  Self-published, available
through Lindsey Pubs., 1994.

Martin, T.C. The Inventions, Researches and Writings of Nikola Tesla.
 Reprinted by Barnes & Noble,  1894.

Hope this helps!  Good luck experimenting!

//    S     //

TIMOTHY RANEY, TCBOR

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