Martin,
  I disagree with two points you made in your reply to Marchal.  1.  I se=
e
no need to balance the boom around the wire attachment point.  2.  I thin=
k
that using a knife edge for the boom pivot is asking for complications.  =
If
the knife edge is longer than say 1/16", there will be uncertainty as to
the location of the actual point of contact since this will vary as the
boom is positioned up or down from where the knife edge is exactly parall=
el
with its contact surface.  I think that using a ball end (say 1/8" dia.) =
or
a disk is easier.  I have put a photo of my pivot which uses a disk (abou=
t
1 1/2" in dia.) on my web site.     www.webspan.net/~bbarns
The disk simply rests on a flat brass bar.  This makes it easy to positio=
n
the contact since it can be anywhere on the bar.
  Many hardware stores carry 1/2" dia. aluminum rod.  I think that this i=
s
good for the boom.
Bob Barns

_____________________________________________________________________

Public Seismic Network Mailing List (PSN-L)