In a message dated 26/06/01, dlstevens69@........... writes:

> I was reading the suggestions from last week about the different 
> modifications to the basic Lehman design. Up until now I have been using 
> the knife-edge method, but I want to switch. What if I was to string a thin 
> guitar string between my two upright poles, remove the exacto blade from my 
> boom and turn it sideways so that the guitar string fits in the slot? 
> Suggestions/criticisms wanted.

Dear Daniel,

       If I understand correctly, the flat ended knife slot would then be up 
against the wire? Otherwise wouldn't you be left with a blunt and poorly 
defined end rocking over the wire instead of a knife edge on a flat? If the 
wire was really taught, this flat would provide a strong force to centre the 
beam and you would probably be unable to get a 15 sec period. If the wire was 
slack to the point of being a loop, the bending stresses of the wire near the 
end of beam would still tend to centre it. If the two wires were bent around 
the end of the slot, I am unsure how accurately you could do it to remove any 
residual bending moments, you would still have a rocking motion at the end of 
the arm and also two wires to bend. If you get pleasure from experimenting, 
why not try it for a limited period and see if you can get it to work? Find 
out what maximum stable period you can get? If you just want a seismometer 
that works well, the two 'standard' solutions are a single short wire under 
tension which is used on commercial instruments, or a ball bearing rolling 
over a hard polished flat, which may be easier for you to make. 
       Good luck! Chris    

In a message dated 26/06/01, dlstevens69@........... writes:



I was 
reading the suggestions from last week about the different 

modifications to the basic Lehman design. Up until now I have been using 

the knife-edge method, but I want to switch. What if I was to string a thin 

guitar string between my two upright poles, remove the exacto blade from my 

boom and turn it sideways so that the guitar string fits in the slot? 

Suggestions/criticisms wanted.




Dear Daniel,



       If I understand correctly, the flat ended knife slot would then be up 

against the wire? Otherwise wouldn't you be left with a blunt and poorly 

defined end rocking over the wire instead of a knife edge on a flat? If the 

wire was really taught, this flat would provide a strong force to centre the 

beam and you would probably be unable to get a 15 sec period. If the wire was 

slack to the point of being a loop, the bending stresses of the wire near the 

end of beam would still tend to centre it. If the two wires were bent around 

the end of the slot, I am unsure how accurately you could do it to remove any 

residual bending moments, you would still have a rocking motion at the end of 

the arm and also two wires to bend. If you get pleasure from experimenting, 

why not try it for a limited period and see if you can get it to work? Find 

out what maximum stable period you can get? If you just want a seismometer 

that works well, the two 'standard' solutions are a single short wire under 

tension which is used on commercial instruments, or a ball bearing rolling 

over a hard polished flat, which may be easier for you to make. 

       Good luck! Chris