where εl and εt are the longitudinal and transversal strain,
respectively. The Poisson ratio is - εl / εt. This ratio varies
from 0 to 0.5 for most materials. The rare auxetic materials are
exceptions; they have a negative Poisson ratio.
The electrical resistance of a conductor (in our case, the
strain gauge) is a function of its length and cross-sectional area.
Therefore, if its length increases and its area decreases, the
resistance will increase. This is established by the well known
Ohm’s law for resistivity, R: