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Re: jumping ring demo



Check the article "A Classroom Jumping Ring," by Carl Schneider and John
Ertel in American Journal of Physics, Volume 66, Issue 8 pp. 686-692
(August 1998). It tells in detail how to construct a good jumping ring.
The jumping ring in this article formed the basis for the design of the
jumping ring apparatus now being marketed by PASCO. Here is the abstract:

Abstract
We present the design of a compact alternating current jumping ring
apparatus which can electromagnetically launch conducting rings across a
classroom. Jump energy and height are calculated for core and thin ring
length, radius, thickness, and material. The effects of core saturation,
permeability, hysteresis and demagnetizing field, ring shielding, and
convective derivative are described. Rings with even small phase lag can
pass by the magnetic pole of the primary in one-quarter of a cycle. Large
ring size relative to skin depth delays the ring current close to the 180
deg of Lenz's law. Contactless jump height measurement enables ring
resistivity determination. Demonstrating the effects of voltage, frequency,
conductivity, permeability, and geometry contributes to understanding
electromagnetism in the classroom. ©1998 American Association of Physics
Teachers.

Gene

Also, there was a jumping ring entered into the 1999 AAPT apparatus
competition. Construction details are available at:

<http://www.rose-hulman.edu/~moloney/AppComp/1999Entries/entry02.htm#RngShoo
ter>

I have tried to build one of the Jumping ring demo's and have met with
some difficulty. Has any one actually built one of these devices? If so,
can you tell me the specifics or point me in the right direction?

Here is what I did.

The coil is made of 900 turns of 16 ga. magnet wire wrapped on a 21 cm
long cylinder with a 2.7 cm inside diameter. A bundle of soft iron wire
is used as a core and is extended 45 cm out the top of the coil. The coil
is connected to 120 VAC and an aluminum ring is placed over the core.
There is only a slight levitation of the ring. (2-3 cm above the coil).

The following measurements were made.
DC Resistance of the coil 3.2 ohms
Inductance of the coil 17 mH
AC Current when connected to 120 V and core fully inserted 5.2 Amps
Magnetic field with the core centered 300 gauss
Magnetic field with the core flush to the coil 1200 gauss

I would like to be able to cause the ring to be propelled into the air as
I have seen other coils of this type do. Any help is appreciated.

Any comments dealing with the reason the ring is repelled from the
apparatus is also appreciated. It seems I know less about this today than
I thought I did last week.

PL

P. Lohstreter "If we want to solve a problem
that
US Science we have never solved before,
Chemistry & Physics we must leave the door to the
unknown ajar."
The Hockaday School
Richard Feynman

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