In a message dated 4/29/02 7:08:54 PM GMT Daylight Time, dcrice@............ 
writes:  (See Below)

Hi Gang,

I have one of these you can have for $50, which is the first bid on the eBay 
instrument. Mine, however, is the complete instrument with the very sensitive 
galvanometer Doug mentions that needs no electronics. It also comes with the 
lamp and optics to focus the light beam on the galvanometer's mirror and 
reflect it from there to the light sensitive paper on the drum. The length of 
the light beam is 1 meter. It is a complete instrument made by Sprengnether 
Instrument Company in St. Louis, Missouri, USA. It is packed in its original 
wooden shipping box and has never been unpacked. I did open the box to make 
sure everything is there. It is, including the owners manual. The 
galvanometer and light beam optics are wrapped in pages from the 23 July 
1973, St. Louis Dispatch newspaper. I am old (84) and would like to find a 
good home for this beautiful precision instrument because I will not live 
long enough to do anything with it. There are too many other things I must do 
before my time comes and I must go. If you are interested I can send pictures 
copied from the owner's manual.

Cap Hossfield
935 Franklin Turnpike
Hewitt, New Jersey 07421 
USA
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<< Subj:     Re: recording drum on ebay
 Date:  4/29/02 7:08:54 PM GMT Daylight Time
 From:  dcrice@............ (Doug Crice)
 Sender:    psn-l-request@..............
 Reply-to:  psn-l@..............
 To:    psn-l@..............
 
 This is a drum recorder from a classic seismograph vault.
 Back in the olden days, seismometers were connected to an
 optical galvanometer, which is kind of an analog voltmeter
 with a tiny mirror instead of a needle.  They are quite
 sensitive, and a small signal causes the mirror to twist. 
 In the vault, there is a lamp which shines a light beam on
 the mirror then back to the drum.  With perhaps a 4 meter
 optical path, you get optical leverage (or amplification),
 so it's possible to make a pretty sensitive seismograph with
 no electronics at all.
 
 The drum is loaded with photographic paper, and the light
 writes a classic drum recording on the paper (which of
 course has to be developed). To translate the traces each
 revolution, the drum has to move sideways, which is why the
 photo shows the device as being much wider than the actual
 drum.
 
 When the technician comes by daily, he changes the paper,
 develops the record, and re-positions the drum at the start
 point (besides checking the equipment).
 
 If somebody has a pen motor laying around, you could make it
 into a modern pen-and-ink system, and it might be useful for
 that.
 
 
 -- 
 Doug Crice          http://www.georadar.com
 19623 Via Escuela Drive              phone 408-867-3792
 Saratoga, California  95070  USA   fax 408-867-4900 >>
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