Jim,

It starts with the need for a period of 10 seconds or so, if you want to
record distant events.  The Lehman design used a boom of about 2 to 3 feet
in length, with about a 5 pound lead weight on the end, supported by a
suspension wire at an angle of about 30 degrees above horizontal will let
you approach the 10-15 second period with simple adjustements of the
leveling screws.

My first lehman had a 36" long base, about 12" wide made of 2" thick oak.
(No metalworking skills here).  Today you'd probably try to scrounge a
piece of thick aluminum from a salvage yard.  No ferrous materials allowed
at the other end of the boom of course.  Even screws, nuts and bolts, use
only brass or aluminum.  If you are unsure, check with a magnet.

The uprights were 18" pieces of 1/2" threaded pipe, cross-piece was 4"
long, making a long skinny U (upside down of course).  If you can get a
copy of the 1979 Scientific American issue (July I believe) on microfilm at
your library you can see the design.

The dimensions of your coil will depend on the gap of your magnet, so buy
the magnet before you wind the coil.

Ted



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