## PSN-L Email List Message

Subject: Re: more horizontal seis boom angle and 'L'
From: ChrisAtUpw@.......
Date: Thu, 17 Feb 2000 13:08:13 EST

```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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