PSN-L Email List Message

Subject: Re: School project
From: John or Jan Lahr JohnJan@........
Date: Wed, 29 Nov 2006 08:55:58 -0800

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I've posted pictures of a simple seismic system

http://jclahr.com/science/psn/wooden/
The natural period is a function of the tilt of the spring.  As it
becomes more horizontal (by attaching the top of the spring lower on the
mast) the period becomes longer.  However, if one tries to make the
period too long the pendulum will become unstable and it will be hard to
keep it from drifting to the upper or lower stop.
At 02:10 PM 11/28/2006, you wrote:
Hi Folks,
Some discussion on a simple vertical seismometer got me to thinking about
a possible school project for the kids.  At one time, I found a
simple vertical sensor on the internet, it used pvc pipe and a small
9"? spring.  I built it but never tested it because I had an
idea the coil and magnet were just too undersized.  It probably
works but would require some strong amplification, further more, I have
no idea what the period would be.

I wonder if one could use a "Slinky"  a toy
spring used by kids? They are about 3" in dia. and collapsed about
5" tall.  I think I will get one and suspend it vertically, to
check its suspended length and its natural resonance.  I have no
idea how long the spring would expand under its own weight?
But let's say under 8 feet. If so, maybe it could be hung from the
ceiling and the coil and mass would be near the
floor?
A sensitive seismic system must be
covered to prevent air currents from causing vibrations much larger than
those generated by earthquakes.  A vertical system will have to be
very tall, and thus difficult to enclose.

It just might be something kids would enjoy.

Yes, indeed.  This would demonstrate the
concept of a seismic sensor but would not be very good for detecting
earthquakes.
Q. 1.
If I pull down on a spring and release it, it will move up and down,
until it come to rest, if each cycle is 1 sec, (say a very long spring),
is that its natural period? Yes, one
full cycle is the natural period.
Q. 2. If I put a mass on the end of the spring, will it move
slower?
Yes.  If so,
is that, now its period?     Maybe 2 or 3
seconds?
Q. 3. If I could work out the construction details, what period should I
target for a vertical sensor?
The AS-1 seismic systems have a
natural period of 1.6 seconds.  To see surface waves the electronics
with a period of 20 seconds, the electronics have to greatly boost the
low frequencies.
Q. 4. Are there any websites or information on using long vertical
springs?
I've posted various designs on my web
site:

http://jclahr.com/science/psn/
Keep us posted what you come up with!
John

Many thanks, Ted

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