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RE: Variable Capacitance and Inductance

Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Hi Bart,

>I made a temperature  test today on a small coil (1" x 5"). Inductance
measured before any temp change was 405uH and unchanging on the meter. Using
hair dryer I measured 407uH stable. As I let it cool, it dropped back down -
stable. I then put it in the freezer for a little while. Measured 411uH. It
went back to 405uH at room temp. Capacitance couldn't be measured by the

The coil I used was 4.5" diameter by 9.625" length, sealed with several
coats of polyurethane, and wound with 21 gage magnet wire.  I'm presently
bringing the temperature up to 150F degrees in an oven and will measure the
inductance as it cools to room temperature.

It's done.  When the coil was first removed from the oven, and the windings
had reached a temperature of 150F degrees, the inductance was 4.59mH.  As it
cooled and the windings reached 90F degrees the inductance is still 4.59mH.
(I'm wondering if baking the coil is a method of raising the physical
inductance of a coil?)  At 80F degrees the LCR meter started jumping back
and forth between 4.59mH and 4.58mH.  That's a large temperature span to
effect a change in inductance.  With the coil completely cooled to room
temperature the inductance is steady at 4.58mH.  The soaking from last night
may have contributed to the extra .01mH inductance after the baking.  Time
will tell.

>Thermal expansion as Terry mentioned is all I'm saying. Temp changes both L
and C because the object physically changes - this should be easily
understood. Also, the amount of changes depends on the L and C value based
on the objects size and amount of physical change. Usually, pretty small.

The difference in inductance you measured was much smaller than the change
in inductance that I measured.  This is proportional to the coil size.
Temperature alone cannot explain the larger variations in inductance
measurements reported over the years by various persons, especially since
temperature variations in the coils are likely rarely to exceed 50F degrees.

>Humidity affects walls and other objects which can become greatly more
conductive in humid conditions. If the walls or whatever is conductive or
significantly changed in conductance by water saturation, you're not going
to see a defining change. As far as that goes, the object itself isn't what
is changing, but the capacitive affects of objects to test object (not the
test object itself unless the environment causes a physical change). The
proximity has much to do with how much change you will measure. A huge coil
in a small bath room will show a greater change than a small coil in the
same bathroom (capacitive affects).

Essentially what you are saying then is that humidity alters the negative
ion to positive ion ratio in a given environment?  Obviously the walls are
not conducting in the sense of current.  But the walls could be conducting
in the sense that they change to a greater or lesser charge.  This change in
wall charge is what would affect the capacitance of an object nearby?

As for the measurement of inductance being separate from the inductance of
the physical coil, I take issue with this idea.  Granted, careless
measurements give wrong inductances.  But inductance has NOT been proven to
be a characteristic solely determined by the physical characteristics of the
coil.  It has been proven that the effective inductance of a coil can be
changed without changing the physical characteristics of a coil.  Since the
effective inductance of a coil can be changed, the inductance measurement
will also change.  And this change reflects the working inductance of the
coil.  Variations in inductance measurements can mean the effective (and
thus real) inductance of the coil is variable.

The environment of the coil is just as crucial to the coil's operation as
the coil itself.  The same goes for capacitors.  And since the coil and
capacitor are electromagnetic devices, we should be looking more at the
electromagnetic and electrostatic environment, not merely the secondary
environment of temperature and humidity.