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Subject: Re: Lehman construction
From: ted@..........
Date: Tue, 11 Jan 2000 16:29:46 -0500

If you use a ball bearing or small disk as the base support for the boom,
remember that this single-point support allows the boom to rotate around
its long axis.  In fact if you mount your coil off to one side of the boom
(like the pictures of the Lehman in the original SciAm article) the
off-center mass of the coil applied to the boom will drag it down until the
coil is hanging straight down, or more likely dragging on the base.  I had
to use counterweights to get it to hang straight.  Drilling your 5 lb. lead
weight slightly off center and then using it as a counterweight works
(tighten it when it balances the coil).

I always recommend that people use a "Y" attachment to suspend the boom,
where the two wires join into one just before attaching to the crosspiece.
This has greatly lessened the rotational movement for me - and this is a
source of noise since it causes relative movement of the magnet and coil.

Flexible connectors can resist a torque but I believe it is best not to
have any torque applied to the boom at all.  Sean-Thomas, have you noticed
whether it is really important to make sure the coil and magnet are lined
up with the swing plane of the boom, to eliminate any rotational impulse
imparted by the resistive damping, or is this simply too small to measure?

Regards, Ted


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Larry Cochrane <cochrane@..............>