  A quantitative theory of operation of a novel device, namely Direct String
Magnetic Gradiometer (DSMG), is presented. The paper provides a detailed
analysis of DSMG basic functions and measured quantities, represented in terms
of physical parameters that are known either a priori or can be experimentally
determined. It leaves a reasonable degree of freedom to further investigate
some of the finer detail of this new instrument based on experimental results
coming both from the laboratory environment and from field trials. The analysis
also allows us to quantitatively evaluate the error budget for an optimised
DSMG.
