PSN-L Email List Message

Subject: Re: Long Period Pendulums
From: "Geoff" gmvoeth@...........
Date: Mon, 25 Feb 2008 22:39:15 -0700

Is it possible to use a laser pointer and
surface reflective mirrors to
obtain a mechanical amplification
of motion then simply use a solar cell
as a pickup ?

You might locate a light source in a way
you can not locate any mechanical thing.

To get the most response with the least
mass getting in the way ?

Say using the laser pointer as the mass
and a power wire as the pendulum wire.

Something like that ??

Or the laser pointer attached to
the rod at the top of the pendulum ?

get the mirrors on each side so
you set up a bouncing beam of maybe
50 feet ?

The mirrors are the biggest problem
I bet quality surface silvered 
mirrors are expensive.

----- Original Message ----- 
Sent: Monday, February 25, 2008 9:20 PM
Subject: Re: Long Period Pendulums

> In a message dated 2008/02/23, tchannel1@............ writes:
>> Hi Folks,  This is a 
>> description of different long period pendulums.   My goal in building sensors 
>> is to obtain the long period, like 20 seconds,  In most cases I have found 
>> this hard to do.   I know it can be done, but even with the Lehman the setup to 
>> get to 20 second is, for me difficult.   I usually give up at about 15 
>> seconds.
> Hi Ted,
>       You WON'T get long periods unless you use a good suspension system and 
> magnetic damping. Knife edge and point in a cup suspensions will PREVENT you 
> from getting long periods.
>>  After thinking about the "Rocking Chair Pendulum" recently discussed, I 
>> saw a similarity between it and one of these Long period pendulums, so I went 
>> back to the shop and made another mock up of the "Nearly Balanced Pendulum"    
>> This time I took a little more time.  Using a 48" dowel, I drilled a pivot 
>> hole in the center, inserted a finishing nail into the pivot hole, attached 
>> the nail to a table top, so the dowel would spin like a propeller.   Next I 
>> threaded a 1/2" machine bolt, nut onto one end of the dowel.   I timed the 
>> period at 2.5 seconds.   I threaded another nut on to the opposite end of the 
>> dowel, this time  one half the distance, from the end to the pivot.   I timed the 
>> period at about 3.5 seconds.   I keep moving the second nut closer to the 
>> end of dowel, to match the first nut which remained at the opposite end of the  
>> dowel.   As I move the nut the period got longer and longer.  When I reached 
>> the end of the dowel, I got a 20 second period, with very little effort.
>> All this may be old news to some of you, but I found it to be amassing.   
>> Now I need to build another sensor, using this approach.
>       What do you want to achieve? You can get long periods this way, but it 
> is near useless as a seismic sensor.
>       If you drill the dowel and fit a horizontal round rod at right angles 
> rolling on two horizontal round rods, you can get very long periods with very 
> low hysteresis / offset.
>    Regards,
>    Chris Chapman

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