In a message dated 27/05/2010, tchannel@............ writes:
_http://sismordia.blogspot.com/search/label/Sunday%20seismometer_
(http://sismordia.blogspot.com/search/label/Sunday%20seismometer)
One question for the group. What was the reasoning behind the very large
masses. The use of large masses seems to be repeated. Thanks, Ted
Hi Ted,
These seismometers were all designed before the days of electronic
amplifiers. Two main detection systems were in operation to supply some signal
gain. The early mechanical seismometers relied for gain on coupled lever
systems, usually driving a point scriber sliding on smoked paper or smoked
glass. The maximum gain achieved was about x200, but some were x10 or less.
Air and oil damping systems were used. Early seismometers were not often
damped. Due to the inevitable friction / stiction in the lever systems, a
very large driving mass was required to give an adequate performance at a high
gain. Mirror systems were used in seismoscopes, but increasing the
sensitivity by using multiple reflections does not seem to have been tried.
The Russian / Galitzine seismometers post 1903 were physically much
smaller. Note that the horizontal seismometer is also shown. They used coil +
magnet detection systems driving a long period galvanometer with a flat
surface silvered mirror. The mirror reflected a beam of light to a focus on a
drum of photographic paper in a blacked out room. Some of the gain was
electrical and some was optical, but the combined gains were more than
adequate. Magnetic induction velocity damping was used on these lower mass
systems. But the photographic paper needed to be developed, fixed and dried before
it could be scanned and measured. This is a fairly expensive process with
commercial paper and chemicals and it takes a significant amount of
operator time, but it was widely used before the development of semiconductor
electronics in the 1970's. I wonder what happened to the early method of
producing photo sensitive paper by dipping ordinary paper in Silver Nitrate
solution?
The development of long period vertical seismometers was rapid after
the work of LaCoste on zero length springs in the mid 1930's and the
development of Elinvar low Tc alloy springs.
Curiously, although good galvanometers were available before 1880,
they were first applied in 1903 to the Russian seismometers. The use of
optical systems did enable the construction of small, high sensitivity
instruments. Type history - seismology - seismometer - seismograph etc into Google? I
like the reference, since you can adjust the enlargement.
Regards,
Chris Chapman
In a message dated 27/05/2010, tchannel@............ writes:
One question for the group. What=
was the=20
reasoning behind the very large masses. The use of large masses seems to=
be=20
repeated. Thanks,=20
Ted
Hi Ted,
These seismometers were all designed before=
the=20
days of electronic amplifiers. Two main detection systems were in operatio=
n to=20
supply some signal gain. The early mechanical seismometers relied for gain=
on=20
coupled lever systems, usually driving a point scriber sliding on smo=
ked=20
paper or smoked glass. The maximum gain achieved was about x200, but=
some=20
were x10 or less. Air and oil damping systems were used. Early seismo=
meters=20
were not often damped. Due to the inevitable friction / stiction in=
the=20
lever systems, a very large driving mass was required to give an adeq=
uate=20
performance at a high gain. Mirror systems were used in seismoscopes, but=
=20
increasing the sensitivity by using multiple reflections does not seem to=
have=20
been tried.
The Russian / Galitzine seismometers post 190=
3 were=20
physically much smaller. Note that the horizontal seismometer is also show=
n.=20
They used coil + magnet detection systems driving a long period=20
galvanometer with a flat surface silvered mirror. The mirror reflecte=
d a=20
beam of light to a focus on a drum of photographic paper in a blacked=
out=20
room. Some of the gain was electrical and some was optical, but the=
=20
combined gains were more than adequate. Magnetic induction velocity dampin=
g was=20
used on these lower mass systems. But the photographic paper needed to be=
=20
developed, fixed and dried before it could be scanned and measured. This=
is a=20
fairly expensive process with commercial paper and chemicals and it takes=
a=20
significant amount of operator time, but it was widely used befo=
re the=20
development of semiconductor electronics in the 1970's. I wonder what happ=
ened=20
to the early method of producing photo sensitive paper by dipping ordinary=
paper=20
in Silver Nitrate solution?
The development of long period vertical=20
seismometers was rapid after the work of LaCoste on zero length=
=20
springs in the mid 1930's and the development of Elinvar low Tc alloy=20
springs.
Curiously, although good galvanometers were=
=20
available before 1880, they were first applied in 1903 to the Russian=20
seismometers. The use of optical systems did enable the construction=20
of small, high sensitivity instruments. Type history - seismology -=
=20
seismometer - seismograph etc into Google? I like the reference, since you=
can=20
adjust the enlargement.
Regards,
Chris Chapman