## PSN-L Email List Message

Subject: Re: Lehmann
From: 1goss@...........
Date: Sat, 02 Apr 2005 02:16:13 +0000

```Thanks for the reply Chris I will post pictures when I get it all going.

Bryan S Goss

In a message dated 02/04/2005, 1goss@........... writes:

> I have built a Lehman seismograph using the 1/2 pipe and using the pipe
> union for a pivot point.
>  My question is when setting the period for the 30 inch boom. Is that from
> left to right then back to left if this is the case I am only getting to about
> 10 sec also should the boom be level? Should the gate effect take care of
> the swing back to centre.

Hi Bryan,

The period is for a complete oscillation, say from when it is central,
to one extreme, to the other extreme and then back again to the centre. The
base levelling screws should be set up so that the pendulum comes back to the
central position.

The boom itself does not have to be level. It is the small angle
between the true vertical and the line joining the top and bottom pivot points
which is critical.

You can make a good bottom pivot using a ball bearing. See Frank
Cooper and John Cole http://pages.prodigy.net/fxc/ and
http://pages.prodigy.net/fxc/JC.html

It is quite easy to make a magnetic damper with 4 NdFeB magnets and an
Al or Cu plate.

Regards,

Chris Chapman

In a message dated 02/04/2005, 1gos=
s@........... writes:

I have built a Lehman seismogra=
ph using the 1/2 pipe and using the pipe union for a pivot point.
My question is when setting the period for the 30 inch boom. Is that from l=
eft to right then back to left if this is the case I am only getting to abou=
t 10 sec also should the boom be level? Should the gate effect take care of=20=
the swing back to centre.

Hi Bryan,

The period is for a complete oscillatio=
n, say from when it is central, to one extreme, to the other extreme and the=
n back again to the centre. The base levelling screws should be set up so th=
at the pendulum comes back to the central position.

The boom itself does not have to be lev=
el. It is the small angle between the true vertical and the line joining the=
top and bottom pivot points which is critical.

You can make a good bottom pivot using=20=
a ball bearing. See Frank Cooper and John Cole http://pages.prodigy.net/fxc/ and http://pages.prodigy.net/fxc/JC.html

It is quite easy to make a magnetic dam=
per with 4 NdFeB magnets and an Al or Cu plate.

Regards,

Chris Chapman

```