PSN-L Email List Message

Subject: idea for an axis
From: Randall Peters PETERS_RD@..........
Date: Thu, 08 Nov 2007 11:57:49 -0500

Since the VolksMeter uses tungsten carbide to establish the axis (extracted from
ball-point pens), I should have thought of the following a long time ago.
   A key to reducing rolling friction is to work with hard surfaces.  Another key
to reducing friction in general (if possible) is to reduce the normal force.  Both
are achievable by hanging a pendulum from a rare earth magnet, using the ferrous
property of the tungsten carbide.
    In a brief experiment this morning I stuck a 1/2 in cylindrical rare earth
magnet to the top of a steel door frame and then hung a ball point pen from the
magnet.  Discovered that the tungsten carbide tip of the pen could support about
100 grams of weight.  Of course this arrangement is unsatisfactory for a
seismometer because the physical pendulum that results (swinging pen) moves as a
spherical pendulum.
    To get the required planar motion I took the refills of two pens and glued
them together.  The pair of pen points can support about 200 grams of an inertial
mass while constrained to motion in a plane.
    The quality factor of this oscillator proved to be really high, with the unit
swinging in observable free decay for many hundreds of cycles.  It is clear then,
that the friction is very small indeed, by (i) taking advantage of the hardness of
both the magnet and the small tungsten carbide balls; and (ii) because the field
gradient of the magnet provides support for much of the mass of the pendulum, so
that the normal force is reduced as compared to most other configurations.
     For you folks who have played with various axis types, what do you think
about this?

[ Top ] [ Back ] [ Home Page ]