# Re: 130kW Coil Update [helium gap]

From: 	Edward V. Phillips[SMTP:ed-at-alumni.caltech.edu]
Sent: 	Thursday, December 11, 1997 1:35 PM
To: 	tesla-at-pupman-dot-com
Subject: 	RE: 130kW Coil Update [helium gap]

"Alas, the gap is of the open frame type, far from airtight.
As I understand it, helium actually has a lower heat capacity,
since monatomic elements only have 5 degrees of freedom,
where diatomic ones have 7, allowing them to carry more heat
energy per deg C.  That's why He compressors run so hot.

At STP, I believe that helium gas has a much lower breakdown
voltage than dry N2.  But is this good or bad?

-GL
"
According to the Handbook of Chemistry and Physics,
at -180 degrees C the specific heat of He is 1.25, and that of
H2 is 2.64.  They don't give a value for He at room temperature,
but H2 at 15 deg C has a specific heat of 3.389. N2 at
15 deg C the value is 0.2477.  Units for all are cal/g.  On
the basis of those numbers, would thin He would be a much
better coolant.  (I have read that divers using He-O2 atmosphere
at high pressures have problems withstaying warm.)
As for the open structure, I can't resist telling an old story.
Once upon a time (almost 50 years ago) I was trying to spin a
disk about 36" diameter with a 5 HP syc motor.  In theory it
should have worked, but in practice the motor "wouldn't pull
in".  Someone suggested running in Helium, so we but a
light wooden frame around the whole setup, motor and all,
and filled it with He.  As soon as the motor was started it started
pumping air out the periphery and "sucking" air in at the middle.
Net result was whole structure collapsed, got caught in the
disk, and was thrown violently all over the lab.  Fortunately,
no one got hurt but that was the end of that experiment.
As for the lower breakdown voltage, I would think that
would be a serious problem, as it would take a larger gap
opening to quench the discharge.
Ed