## PSN-L Email List Message

Subject: Re: Determining Mass Size for Lehman Seismometer
From: ChrisAtUpw@.......
Date: Mon, 31 Mar 2008 22:38:55 EDT

```In a message dated 2008/04/01, gel@................. writes:

> How do I determine the mass size of the pendulum for a Lehman seismometer.
> Is there a method to the madness?
> Gary

Hi Gary,

There is method in the madness!

The period of a pendulum is independant of the mass, since the
gravitational attraction and the moment of inertia both depend on M !

But you need ~the maximum radius of gyration, so you make the mass
several times larger than the weight of the arm on it's own. But you don't want
to stress the suspension too much.  A mass of 1 to about 5 lbs is generally OK.
Lead is dense, but difficult to fasten, so I generally use brass. Avoid iron,
which is magnetic.

Hope that this helps!

Regards,

Chris Chapman
In a me=
ssage dated 2008/04/01, gel@................. writes:

How do I determine the mass siz=
e of the pendulum for a Lehman seismometer. Is there a method to the madness=
?
Gary

Hi Gary,

There is method in the madness!

The period of a pendulum is independant=
of the mass, since the gravitational attraction and the moment of inertia b=
oth depend on M !

But you need ~the maximum radius of gyr=
ation, so you make the mass several times larger than the weight of the arm=20=
on it's own. But you don't want to stress the suspension too much.  A m=
ass of 1 to about 5 lbs is generally OK. Lead is dense, but difficult to fas=
ten, so I generally use brass. Avoid iron, which is magnetic.

Hope that this helps!

Regards,

Chris Chapman
```