From: "Connie and Jim Lehman" lehmancj@...........

Date: Tue, 13 Jul 2004 10:12:39 -0400

T.Dick--thanks for your inquiry. I can only comment on the original = design and how we arrived at the mass. The first model had a boom of = 140 cm and a mass of 4kg or about 9 lbs.--empirical all the way. Those = early Jesuit "cantilever" systems used much heavier masses, as they were = friction linked to give a mechanical readout on smoked paper, but later = , a reflected beam of light on photographic paper reduced the need for = such a heavy mass. The mass is related to the natural period of swing somewhat--(even = though different masses of a bob on a free hanging pendulum makes little = difference in period). More important--the mass must be = supported...The support wire at the angle with horizontal suggested must = be strong. If you do the arithmetic, a 5 lb mass will tug a tension of = between 30 & 40 lbs on the upright hinge area when the angle is 30-40 = degrees. ((Now if you want a real puzzle. As that angle goes to zero, = the tension approaches infinity)) When we made the final design as published, a 5 lb mass was suggested = as 5 lb "bricks" of lead were available. We reduced the boom to fit = everything under a 1 meter box. The physical sensor worked so well with = set up procedure, stability and performance we haven't deviated from = those parameters. That is not saying other parameters won't work. With = care no doubt the physical size of the "Lehman" could be halved, but = instabilites no doubt would frustrate the user. On a bit of related technology. Several years ago we built a = portable Foucault pendulum on a 4-ft tripod, drove it magnetically from = below, and achieved 5% accuracy. We thought this was pretty neat, until = a gentleman from Michigan built a good working model about 12 inches = high. One never quite knows where the empirical approach in mechanical = devices take one!! Best Wishes--Jim Lehman ----- Original Message -----=20 From: tdick=20 To: psn-l@................. Sent: Monday, July 12, 2004 9:09 PM Subject: feasible maximum mass for a Lehman seismometer At the risk of ruining a good discussion going on the last couple of = days -- I would like to have some input on what the best/maximum mass = for Lehman horizontal seismometerT.Dick--thanks for your inquiry. = I can only=20 comment on the original design and how we arrived at the mass. The = first=20 model had a boom of 140 cm and a mass of 4kg or about 9 lbs.--empirical = all the=20 way. Those early Jesuit "cantilever" systems used much heavier = masses, as=20 they were friction linked to give a mechanical readout on smoked paper, = but=20 later , a reflected beam of light on photographic paper reduced the need = for=20 such a heavy mass.The mass is related = to the=20 natural period of swing somewhat--(even though different masses of a bob = on a=20 free hanging pendulum makes little difference in period). = More=20 important--the mass must be supported...The support wire at the angle = with=20 horizontal suggested must be strong. If you do the arithmetic, a 5 = lb mass=20 will tug a tension of between 30 & 40 lbs on the upright hinge = area=20 when the angle is 30-40 degrees. ((Now if you want a real = puzzle. As=20 that angle goes to zero, the tension approaches infinity))When we made the final = design as=20 published, a 5 lb mass was suggested as 5 lb "bricks" of lead were=20 available. We reduced the boom to fit everything under a 1 meter=20 box. The physical sensor worked so well with set up procedure, = stability=20 and performance we haven't deviated from those parameters. That is = not=20 saying other parameters won't work. With care no doubt the = physical size=20 of the "Lehman" could be halved, but instabilites no doubt would = frustrate the=20 user.On a bit of related=20 technology. Several years ago we built a portable Foucault = pendulum on a=20 4-ft tripod, drove it magnetically from below, and achieved 5%=20 accuracy. We thought this was pretty neat, until a gentleman from = Michigan=20 built a good working model about 12 inches high.One never quite knows = where the=20 empirical approach in mechanical devices take one!! Best = Wishes--Jim=20 Lehman----- Original Message -----From:=20 tdickSent:Monday, July 12, 2004 = 9:09 PMSubject:feasible maximum mass = for a=20 Lehman seismometerAt the risk of ruining a good = discussion going on=20 the last couple of days -- I would like to have some input on what the = best/maximum mass for Lehman horizontal=20 seismometer