## PSN-L Email List Message

Subject: Re: feasible maximum mass for a Lehman seismometer
From: "Connie and Jim Lehman" lehmancj@...........
Date: Tue, 13 Jul 2004 10:12:39 -0400

```T.Dick--thanks for your inquiry.  I can only comment on the original =
design and how we arrived at the mass.  The first model had a boom of =
140 cm and a mass of 4kg or about 9 lbs.--empirical all the way.  Those =
early Jesuit "cantilever" systems used much heavier masses, as they were =
friction linked to give a mechanical readout on smoked paper, but later =
, a reflected beam of light on photographic paper reduced the need for =
such a heavy mass.
The mass is related to the natural period of swing somewhat--(even =
though different masses of a bob on a free hanging pendulum makes little =
difference in period).  More important--the mass must be =
supported...The support wire at the angle with horizontal suggested must =
be strong.  If you do the arithmetic, a 5 lb mass will tug a tension of =
between 30 & 40 lbs  on the upright hinge area when the angle is 30-40 =
degrees.  ((Now if you want a real puzzle.  As that angle goes to zero, =
the tension approaches infinity))
When we made the final design as published, a 5 lb mass was suggested =
as 5 lb "bricks" of lead were available.  We reduced the boom to fit =
everything under a 1 meter box.  The physical sensor worked so well with =
set up procedure, stability and performance we haven't deviated from =
those parameters.  That is not saying other parameters won't work.  With =
care no doubt the physical size of the "Lehman" could be halved, but =
instabilites no doubt would frustrate the user.
On a bit of related technology.  Several years ago we built a =
portable Foucault pendulum on a 4-ft tripod, drove it magnetically from =
below, and achieved 5% accuracy.  We thought this was pretty neat, until =
a gentleman from Michigan built a good working model  about 12 inches =
high.
One never quite knows where the empirical approach in mechanical =
devices take one!!  Best Wishes--Jim Lehman
----- Original Message -----=20
From: tdick=20
To: psn-l@.................
Sent: Monday, July 12, 2004 9:09 PM
Subject: feasible maximum mass for a Lehman seismometer

At the risk of ruining a good discussion going on the last couple of =
days -- I would like to have some input on what the best/maximum  mass =
for Lehman horizontal seismometer

I can only=20
comment on the original design and how we arrived at the mass.  The =
first=20
model had a boom of 140 cm and a mass of 4kg or about 9 lbs.--empirical =
all the=20
way.  Those early Jesuit "cantilever" systems used much heavier =
masses, as=20
they were friction linked to give a mechanical readout on smoked paper, =
but=20
later , a reflected beam of light on photographic paper reduced the need =
for=20
such a heavy mass.
The mass is related =
to the=20
natural period of swing somewhat--(even though different masses of a bob =
on a=20
free hanging pendulum makes little  difference in period).  =
More=20
important--the mass must be supported...The support wire at the angle =
with=20
horizontal suggested must be strong.  If you do the arithmetic, a 5 =
lb mass=20
will tug a tension of between 30 & 40 lbs  on the upright hinge =
area=20
when the angle is 30-40 degrees.  ((Now if you want a real =
puzzle.  As=20
that angle goes to zero, the tension approaches infinity))
When we made the final =
design as=20
published, a 5 lb mass was suggested as 5 lb "bricks" of lead were=20
available.  We reduced the boom to fit everything under a 1 meter=20
box.  The physical sensor worked so well with set up procedure, =
stability=20
and performance we haven't deviated from those parameters.  That is =
not=20
saying other parameters won't work.  With care no doubt the =
physical size=20
of the "Lehman" could be halved, but instabilites no doubt would =
frustrate the=20
user.
On a bit of related=20
technology.  Several years ago we built a portable Foucault =
pendulum on a=20
4-ft tripod, drove it magnetically from below, and achieved 5%=20
accuracy.  We thought this was pretty neat, until a gentleman from =
Michigan=20
built a good working model  about 12 inches high.
One never quite knows =
where the=20
empirical approach in mechanical devices take one!!  Best =
Wishes--Jim=20
Lehman

----- Original Message -----
From:=20
tdick
To: psn-l@..............
Sent: Monday, July 12, 2004 =
9:09 PM
Subject: feasible maximum mass =
for a=20
Lehman seismometer

At the risk of ruining a good =
discussion going on=20
the last couple of days -- I would like to have some input on what the =

best/maximum  mass for Lehman horizontal=20
seismometer
```