PSN-L Email List Message

Subject: Re: recording drum on ebay
From: Doug Crice dcrice@............
Date: Mon, 29 Apr 2002 11:08:08 -0700

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

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

Doug Crice
19623 Via Escuela Drive		      phone 408-867-3792
Saratoga, California  95070  USA	fax 408-867-4900

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