PSN-L Email List Message

Subject: Re: Fw: John Cole's Mini-Mini Lehman
From: CapAAVSO@.......
Date: Sun, 18 Jun 2000 18:33:47 EDT

In a message dated 6/16/00 1:34:37 PM Eastern Daylight Time, 
mlamb1@.......... writes:

<< I think its a kind of attachment to the past sometimes, but if the
 kids are drawn to the drums, I can't really explain it, except for
 possible the drums being physically real and the computer
 monitors aren't quite the "real" thing?
Hi Meredith,
    It may be that they are used to seeing video games and TV on a flat 
screen and this is so different. It's a three dimensional thing doing 
something familiar to all, a pen drawing a line on a piece of paper. I have 
always recorded on a drum myself and it is much more satisfying. I had it set 
up in the kitchen on the counter next to the refrigerator (You gotta live 
alone to get away with this !!! ). Sometimes I'd be there when a quake 
started and I could watch it come in. When I got up in the morning the 
night's activity was right there to see at a glance. I'd come home at the end 
of the day and it was all there to see without having to fire up a computer 
and scroll through a digital recording. A seismograph drum is one analog 
devise that can't be replaced with digital technology without loosing a lot 
of the fascination of recording earthquakes. 
    It is surprising more amateurs don't record on drums. I guess they would 
if they could find one. Actually it is not all that difficult to build your 
own. If you have built your own Lehman you have probably developed enough 
skill with simple hand tools to build a seismograph drum. You will say, "how 
can I make the drum if I don't have a well equipped machine shop in my 
basement or garage?". I'll tell you how. Go to your nearest Hobby Shop that 
caters to adults who build radio controlled airplanes. Browse around and 
you'll see all kinds of handy stuff to build all kinds of things that are not 
necessarily airplanes. See their supply of model airplane plywood. This comes 
in various thickness and in sheets 12-inchs wide by 48-inchs long. Check out 
their thinnest piece. It will be only 1/64-inch thick and yet three-ply 
Birch, usually made in Finland. Just what you need to wrap into a seismograph 
drum !! But you will need a frame to wrap it on. Buy also two pieces 1/8-inch 
thick, 5-ply, Birch plywood to cut out some circles for a frame. Buy some 
1/2-inch square Balsa wood to make spacers for the frame. Also a little model 
makers miter box and saw to cut the spacers square. You will also need the 
model airplane makers best friend, Cyanoachrylate glue, to stick all this 
stuff together. Get the gap filling type that sets in about 30 seconds.
    The shaft for your drum can be a 3/4-inch wooden dowel that you will 
slide the frame onto. This will require 3/4-inch holes in the 1/8-inch thick 
plywood disks for the frame. To drill these holes you will need a 3/4-inch 
"Forstner" drill from Home Depot. A Forstner drill is the only way to drill a 
clean hole. Any other way will make a splintery mess. You will of course need 
an electric hand drill, preferably a cordless one, to put the 3/4-inch 
Forstner drill in. A compass to draw the diameter circle on the disk should 
be made from a strip of the 1/64-inch plywood. Cut off a piece about one half 
inch wide with ordinary scissors. With a pin, punch two holes at the ends. 
One hole will pin the compass to the plywood and the other is for the point 
of your pencil. Now is the time to decide what diameter you want to make the 
drum. I recommend a diameter that will give a circumference of 900 
millimeters. If you then drive the drum at a speed that makes one revolution 
in 15 minutes the paper under the pen will move exacty one millimeter per 
second. You can then measure the distance from a minute mark to the first 
impulse of an earthquake with a flexible machinist's scale graduated in 
millimeters. There will be 60 millimeters between minute marks so you can 
count the seconds to the first impulse accurate to a few tenths of a second. 
    Lay two pieces of the 1/8-inch plywood sides by side and draw a circle to 
the computed diameter. The disks will be in two pieces which you later glue 
together. Cut the circle out roughly with a jig saw if you have one, 
otherwise you can do it with the model makers saw. After you have glued the 
two pieces together, file the outside diameter to the circle line you have 
drawn. With an ordinary compass draw a circle 7/8-inch in diameter centered 
on the center for the big circle. Now drill the 3/4-inch hole with the 
Forstner drill, being very careful to start its center point in the compass 
center hole. The finished hole MUST be centered in the 7/8-inch circle. If 
not start over, moving along a couple of inches leaving the wrongly made hole 
off to one side of the 3/4-inch dowel where it does no harm. Fill it with 
Plastic Wood if you like. Make a second disk with a nicely centered hole. Cut 
about 15 Balsa wood spacers each exactly 3 1/2-inches long using the miter 
box so the ends are square. Lay one disk on the bench and glue a ring of 
spacers about 1/2-inch in from the outside edge. Put others inside near the 
shaft hole. Mount them by putting a drop of cyanoachrylate on the disk and 
pressing the spacer squarely into the drop and holding it in place for 20 
seconds. Lay the other disk on a table and set the disk with the spacers on 
top of it. True it up so it is exactly over the lower disk with a square, 
going around the edges several times to get it just right. Put a weight on it 
to hold it in place while you put a drop of cyanoachrylate beside each 
spacer. Capillary action will draw the glue under the spacers. Cut a strip of 
1/64-inch plywood six inches wide with scissors. Lay it on the bench and very 
slowly roll the frame over it gluing it in place as you go. It will usually 
slither off to one side because no matter how careful you are there is apt to 
be a very slight difference in the diameter of the disks and you  will be 
wrapping a slight cone. That's the reason for the 6-inch width. I didn't want 
you to find out about this the hard way. Trim off the excess and file it 
smooth. Make three more like this, stack them on the 3/4-inch wooden dowel 
and you will have have a drum 14-inches long on which you can draw a helical 
line 86.4 meters long in 24 hours. If you drive the pen carrier along 2 1/2 
mm per 15 minute revolution the hour lines will be one centimeter apart and 
you can determine the correct hour line for an earthquake by measuring how 
many centimeters it is from the start line by which you have marked correct 
time and clock error.
    When you have built the drum let me know and I'll tell you how to make 
the rest of the thing. I can also tell you how to make a quartz kitchen clock 
into a precision timer to generate minute marks that will allow you to time 
the first impulse to tenths of a second. 
    If you are not quite up to building your own drum I have an extra one 
I'll sell you for the price of a Mini-Mini Lehman. It was made by 
Sprengnether Instrument Company in St. Louis. It is brand spanking new in its 
original shipping box with the owners manual. It has never been unpacked. 
Some small accessories are wrapped in a July 1972 St. Louis newspaper !! You 
could hook it to your Lehman just as it is and it would record your 
earthquakes on photo sensitive paper. You would have to convert it to pen and 
ink if you wanted to watch it work. It requires no amplifier. The output of 
your Lehman would go to a sensitive precision electrometer, A little mirror 
on the electrometer reflects a light beam onto the photosensitive paper to 
draw a line that records the earthquakes. If anybody is interested, send me 
an email letter and I'll tell you more about it.
Best regards,


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Larry Cochrane <cochrane@..............>