First:
Karl, sorry, but diamagnetism may be present, but the main effect of RF
tuning with brass slugs is due to the shorted turn effect of the slug.
As the slug is brought into the coil it couples a shorted turn to the
coil, thus lowering its effective primary inductance, thereby causing
the resonance frequency to rise.  The key is to use a very conductive
material so the Q doesn=92t take too big a hit.  That is the main drawbac=
k
to the slug, Q has to go down from the coil=92s Q alone, conversely, the
use of ferrite can be accompanied by an increase in Q (as long as the
loss of the ferrite is low in comparison to the loss in the coil). So
decreasing Q can be OK in an oscillator, but generally is at odds with
tuned filter type applications where a constant BW is desired.  So to
maintain constant BW, Q has to increase in proportion to the frequency
(or inductance).

Second:
Meredith Lamb asked, =93=85questions involving using the neodymium magnet=
 as
a seismic sensor=85=94
I knew for a long time that a carbon rod had been incorporated in a
seismic sensor but I didn=92t know much more until recently I finally
found the report with the construction details.  ARPA sponsored Arthur
D. Little company to design a borehole seismic sensor around 1967-69.
The final report is titled:
DEVELOPMENT OF A THREE-AXIS LONG-PERIOD SEISMOGRAPH
Report no. C-69429.  It=92s a 75 page report with many of the constructio=
n
details.  It used a 2.5=94 by 0.086=94 dia. carbon rod approx. 0.8 gms in=

weight suspended diamagnetically as the seismic mass.  They sensed the
position photoelectrically.  The sensor itself was essentially flat from
DC to 0.5 Hz.  With filters they tailored the response to be flat (3dB)
from 0.02 to 0.07 Hz. teleseimic events of mb <=3D 5 were consistently
recorded.  You can order it from NTIS at 800-553-6847 for $34.  (It=92s
public domain, and if you want a copy from me I=92ll do the whole thing
for $10, US postage included.)

Sincerely,
Charles R. Patton
charles.r.patton@........



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