# Controlling Marbles

[From Bruce Abbott (951023.1815 EST)]

Peter Burke (23 Oct 1995 13:01) --

I have found that when I test for a controlled variable in a system of a
marble in the bottom of a bowl, no matter how I disturb the position of the
marble, it always comes to rest at the bottom of the bowl again. I surmise
that the position of the marble is controlled. Now, tell me about
perceptions and comparators and how PCT explains this! Certainly the "test
for a controlled variable" doesn't necessarily tell us very much, even
whether or not we have an ECU.

Peter, the problem is that the marble, gravity, and the bowl do not together
constitute a control system, although they do constitute an equilibrium
system embodying negative feedback. To convert your marble into an
effective control system, you would have to add a mechanism through which a
small error in its position (reference being the bottom of the bowl) would
produce an action on the marble's part strong enough to counter the effect
of your disturbance (although it will never do so completely). As it
stands, the marble does "push back" against your fingers as you lift it up
the side of the bowl (with a force less than or equal to its weight,
depending on where it lies along the side of the bowl), but it does not push
back hard enough to resist your effort to lift it. Thus there is negative
feedback but no amplification. Only when you release the marble will the
potential energy added by your lifting be converted to motion and the
marble, after a number of oscillations, will settle back to the bottom of
the bowl.

Regards,

Bruce

[Peter Burke (23 Oct 1995 16:40)]

Peter, the problem is that the marble, gravity, and the bowl do not together
constitute a control system, although they do constitute an equilibrium
system embodying negative feedback. To convert your marble into an
effective control system, you would have to add a mechanism through which a
small error in its position (reference being the bottom of the bowl) would
produce an action on the marble's part strong enough to counter the effect
of your disturbance (although it will never do so completely). As it
stands, the marble does "push back" against your fingers as you lift it up
the side of the bowl (with a force less than or equal to its weight,
depending on where it lies along the side of the bowl), but it does not push
back hard enough to resist your effort to lift it. Thus there is negative
feedback but no amplification. Only when you release the marble will the
potential energy added by your lifting be converted to motion and the
marble, after a number of oscillations, will settle back to the bottom of
the bowl.

I think it is easy to dismiss the marble in a bowl because we "know" so much
about its behavior. But, it would appear that your criteria has perhaps
shifted a bit from a countering force to an effective countering force.
Would the e-coli described in several previous posts be and ECU under such
criteria if a screen with a mesh fine enough to prevent the e-coli from
passing through, but allowing the nutrient the pass through, were placed
between the e-coli and the nutrient source?
Peter

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Peter J. Burke Phone: 509/335-3249
Sociology Fax: 509/335-6419
Washington State University
Pullman, WA 99164-4020 E-mail: burkep@unicorn.it.wsu.edu
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