Searching for information regarding LaCoste "zero length" springs a year or two ago; I came across the following documents, which may be of some use during this current discussion. 


You will need to retrieve two patents. These patents are accessible as PDFs through a free patent search web site like Free Patents Online; however, you need to create an account, for free. Go to: 


LaCoste (1942) Patent 2293437:
This patent illustrates a variety of different types of zero lengths springs, including both helical tension and spiral springs. A lot of food for thought for amteur instrument builders!


Willmore (1966) Patent 3292145:
This patent covers the design of a tapered leaf spring. Patent was cited in the BSSA 1982 article describing the STS-1 Seismometer.
I made a curved tapered leaf spring, cut out of a flat tempered steel plastering knife using an abrasive cutting disc mounted in a Dremel tool. Hand formed (curved) the metal leaf spring by partially wrapping it around a metal bar clamped in a rigid bench vice. Spring later used to develop a 4-5 second natural period vertical seismometer, with feedback for closed loop period extension.


If you are interested in how LaCoste and Romberg make their zero length tension springs, download the following 3 meg PDF from:
It's the LaCoste and Romberg Instruction Manual for the model G & D Gravity Meters. Starting at page 4-44, under PHYSICS OF THE SENSOR, there are 4 pages of information.  Bob McClure has made an html excerpt of these pages.




Allan Coleman