Jim Chris,  The center balance point is 28.5-inches. I'll be re-installing
both of Lehman's in the next few months and measure the natural period then.
Thanks for the answers.
Regards, Steve Hammond  PSN Aptos, California

-----Original Message-----
From: ChrisAtUpw@....... 
To: psn-l@.............. 
Date: Thursday, February 17, 2000 10:08 AM
Subject: Re: more horizontal seis boom angle and 'L'


>In a message dated 17/02/00 05:57:02 GMT Standard Time,
>shammon1@............. writes:
>>> I keep coming up with more questions as I think more about the problem.
>For example, where is the end of the boom? In my case, the boom is 38
1/4-in
>long tip to tip. I use a 1/4-copper plate for damping and it hangs on a
>L-bracket at the at the end. At the 32-inch point, the upper wire
intersects
>the boom. The coil and magnet are mounted at the 24 3/4-inch point. So in
>your equation, L=which point? The end of the boom or the
> intersection point where the guide wire is mounted? Regards, Steve Hammond
>PSN  Aptos, California  >>
>
>    In a Lehman 'garden gate' system, the distance 'L' is measured from the
>HINGE LINE to the 'CENTRE OF GRAVITY' of the whole weight + boom system. If
>you have a heavy weight and a very light boom and the boom is mounted at
>right angles to the hinge line, it will be roughly the distance from the
>hinge line to the CENTER of the weight. With a more complicated system
having
>a heavy boom, a magnet and a weight, you need to take the boom off and
>holding it horizontal, find the point where it just BALANCES over a
>horizontal knife edge. You can usually get a rough estimate by balancing it
>over your finger. L is then the horizontal distance from the balance point
to
>the hinge line at the end of the beam. So long as the beam makes and angle
of
>90 +/- 8 Degrees with the hinge line, the error is < 1%. [Otherwise you
have
>to calculate the length of the perpendicular from the actual hinge line to
>the centre or mass.]
>
>    I hope that this makes it clear. Chris
>
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