Jim and John
A ball rolling on a flat is fine for the lower pivot point
but my ball is attached to the mast and the flat is a
small polished hardened steel flat attached to
the pendulum.  This way, the pendulum is less sensitive to
the position of the ball on the flat.  The flat is made from the 
handle of a small flat file which was also hardened.
Cheers all
Danie Overbeek. 
locationTo:            psn-l@.............. Date:          Mon, 10 
Jan 2000 21:11:44 -0600 Subject:       Re: Lehman construction
From:          john c cole 
Reply-to:      PSN-L Mailing List 

jim
 i have been reading your mail with interest. i read the reply submitted
by frank and agree with his reply as well as the others. 
  here is what i have to say. eleven to twelve seconds is about all that
you can expect out of the detector  as it is constructed. i could not get
any more out of mine and it was built the same way.
  the time period is controlled by the upper and lower pivot point. even
the smallest amount of friction  or resistance at these two points is
critical . we are talking about extremely small amounts.
  to lower the friction at the lower pivot point, cut the end off
flat,drill a hole in the end of the pendulum ,drill a hole large enough
for a 1/4" ball bearing to fit in. about half of the bearing protruding
from the pendulum. the surface that the ball bearing rest against should
be sanded with no.400 paper then with no600 paper. the smoother the
better. absolutely no holes drilled to keep the pendulum from migrating.
the weight of the coil and the lead weight will keep it in place. it will
never ,never slide out while it is in operation. it will slide off
sometime while it is being adjusted. put a drop of super glue in the hole
before inserting the ball bearing. if you like, put a l;little more
around the edge of the ball bearing after the first has hardened.
  the top pivot point. i use .015 music wire for the lower section up to
the turn buckle . the upper wire from the turnbuckle to the top pivot
point is .010 thousandth. are you with me? the best source for the wire
of the correct diameter is ,your local  music store. the e string of a
guitar is used. that is the first small string. cost,50 cents to a dollar
.. the diameter of the strings aer written on the package .the smaller
diameter on the string at the top the longer the period.
   using this method i can get any time that i desire. i usually run mine
at about sixteen seconds . twelve seconds or even less is ok. i think
that most professional detectors run at fifteen seconds. any higher and
you will be adjusting all the time .
  ps i have four detectors up and running at this time.    
                                                  
                                                      your friend john

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Public Seismic Network Mailing List (PSN-L)


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