PSN-L Email List Message
Subject: Re: seismometer sensitivity--fundamental physics
From: "Geoff" gmvoeth@...........
Date: Fri, 9 Nov 2007 09:49:37 -0700
Then you got to worry about the Earths
Rotation affecting the direction
of the pendulum. ??
----- Original Message -----
From: "Randall Peters"
Sent: Friday, November 09, 2007 6:35 AM
Subject: seismometer sensitivity--fundamental physics
> Since you asked about period extension to increase sensitivity, let me explain something very few seem to understand. The
> sensitivity of a simple pendulum is just like that of every other seismometer; i.e., you can show that it is proportional to the
> square of the period. This can be proven from
> the equations of motion of the various mechanical oscillators in general, but for the pendulum, understanding is trivial.
> As anybody would expect from 'horse-sense', the longer the 'quasi-rigid' pendulum, the greater the sensitivity of the
> instrument if the sensor is placed at the bottom. Because the period of the pendulum is given by 2 pi times the square root of
> the ratio of length to earth field (little g), one
> sees immediately then, from this well known expression, that the sensitivity is proportional to the square of the period.
> So then--to increase your sensitivity, hang as long a rod as you can find, consistent with your house size, and then place your
> sensor at the bottom. There are many possible sensor types to go with this incredibly cheap but probably very effective
> earthquake detector. They could be moire' pattern
> types with white light (incrediby simple) to function as a tsunami detector. Or they might be greatly, greatly sensitive by means
> of the Ronchi approach, who made optical testing famous by means of a coarse grating (much better than the classic knife edge
> test). The Ronchi ruling works with white
> light! Or the sensor might be capacitive in nature like my SDC array. The list of possibilities goes on and on, thus my interest
> in the serendipty that is likely to come out of the listserve.
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